From b70ac7079afa78301d297e6d1f41e28b3614c2a7 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 7 Sep 2023 13:39:54 +0200
Subject: [PATCH 01/13] Capture axis transforms as an actual transform

---
 R/guide-axis.R | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index eac32b2b98..3bd8666c89 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -540,3 +540,51 @@ axis_label_element_overrides <- function(axis_position, angle = NULL) {
     ))
   }
 }
+
+function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
+  if (is.character(fun)) {
+    fun <- as.trans(fun)
+  }
+  if (is.trans(fun)) {
+    if (fun$name == "identity") {
+      return(NULL)
+    }
+    return(fun)
+  }
+  if (is.null(fun) || is.null(limits) || zero_range(limits)) {
+    return(NULL)
+  }
+  if (!is.function(fun)) {
+    cli::cli_abort(paste0(
+      "The {.arg trans} argument must be a {.cls trans} object, ",
+      "a {.field formula} or {.field function}, not {obj_type_friendly(fun)}."
+    ))
+  }
+
+  # Translation between primary and secondary ranges
+  limits_seq <- seq(limits[1], limits[2], length.out = detail)
+  origin_seq <- scale_trans$inverse(limits_seq)
+  trans_seq  <- fun(origin_seq)
+
+  # Test for monotonicity
+  if (!is_unique(sign(diff(trans_seq)))) {
+    cli::cli_abort("The {.arg trans} transformation must be monotonic.")
+  }
+
+  # Deduplicate in the expanded area of the range that can occur if the
+  # transformation is non-monotonic in the expansion. The split ensures
+  # that the middle duplicates are kept.
+  duplicates <- c(
+    !duplicated(trans_seq[seq_len(detail / 2)], fromLast = TRUE),
+    !duplicated(trans_seq[-seq_len(detail / 2)])
+  )
+  origin_seq <- origin_seq[duplicates]
+  trans_seq  <- trans_seq[duplicates]
+
+  trans_new(
+    "secondary_transformation",
+    transform = function(x) approx(trans_seq, origin_seq, x)$y,
+    inverse   = fun,
+    format    = format_format(digits = 3)
+  )
+}

From 3850af4e6dbc1f30b4e7de6b51947b8a1bc4dac4 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 7 Sep 2023 13:40:43 +0200
Subject: [PATCH 02/13] Plumbing for extra parameters

---
 R/guide-axis.R | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 3bd8666c89..fbf8a91d36 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -55,6 +55,12 @@ guide_axis <- function(title = waiver(), check.overlap = FALSE, angle = NULL,
   new_guide(
     title = title,
 
+    # Override settings
+    breaks = breaks,
+    labels = labels,
+    minor.breaks = minor.breaks,
+    trans  = allow_lambda(trans),
+
     # customisations
     check.overlap = check.overlap,
     angle = angle,
@@ -81,6 +87,10 @@ GuideAxis <- ggproto(
 
   params = list(
     title     = waiver(),
+    breaks    = derive(),
+    labels    = derive(),
+    minor.breaks = NULL,
+    trans     = NULL,
     name      = "axis",
     hash      = character(),
     position  = waiver(),

From b0a80dad14544a44eeb7d746cce662a7673a958f Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 7 Sep 2023 13:41:06 +0200
Subject: [PATCH 03/13] Extended `extract_key` method

---
 R/guide-axis.R | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index fbf8a91d36..862e21cb96 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -113,6 +113,61 @@ GuideAxis <- ggproto(
     ticks_length = "axis.ticks.length"
   ),
 
+  extract_key = function(scale, aesthetic, breaks, labels, trans, ...) {
+    limits <- scale$continuous_range
+
+    # Resolve transformation
+    trans  <- function_as_trans(trans, limits, scale$scale$trans)
+    if (!is.null(trans) && scale$is_discrete()) {
+      cli::cli_warn("Cannot use axis transformation with discrete scales.")
+      trans <- NULL
+    }
+
+    if (!is.null(trans) || !is.derived(breaks) || !is.derived(labels)) {
+      # If anything needs to be computed that is not included in the viewscale,
+      # a temporary scale computes the necessary components
+      temp_scale <- ggproto(
+        NULL, scale$scale,
+        trans  = trans %||% scale$scale$trans,
+        limits = if (scale$is_discrete()) scale$get_limits() else limits,
+        breaks = if (is.derived(breaks))  scale$scale$breaks else breaks,
+        labels = if (is.derived(labels))  scale$scale$labels else labels
+      )
+      # Allow plain numeric breaks for discrete scales
+      if (!(scale$is_discrete() && is.numeric(breaks))) {
+        breaks <- temp_scale$get_breaks()
+      }
+    } else {
+      temp_scale <- NULL
+      breaks <- scale$get_breaks()
+    }
+
+    if (length(breaks) == 0) {
+      return(NULL)
+    }
+
+    mapped <- scale$map(breaks)
+
+    if (!is.null(temp_scale)) {
+      labels <- temp_scale$get_labels(breaks)
+    } else {
+      labels <- scale$get_labels(breaks)
+    }
+    if (is.expression(labels)) {
+      labels <- as.list(labels)
+    }
+
+    key <- data_frame(!!aesthetic := mapped)
+    key$.value <- breaks
+    key$.label <- labels
+
+    if (is.numeric(breaks)) {
+      vec_slice(key, is.finite(breaks))
+    } else {
+      key
+    }
+  },
+
   extract_params = function(scale, params, ...) {
     params$name <- paste0(params$name, "_", params$aesthetic)
     params
@@ -598,3 +653,4 @@ function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
     format    = format_format(digits = 3)
   )
 }
+

From df8e93c344a00bbfdde03018902cce88a44012e0 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 7 Sep 2023 13:57:22 +0200
Subject: [PATCH 04/13] Set and document new arguments

---
 R/guide-axis.R    | 24 +++++++++++++++++++++---
 man/guide_axis.Rd | 14 ++++++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 862e21cb96..e5393dde13 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -6,6 +6,14 @@
 #' [scale_(x|y)_discrete()][scale_x_discrete()].
 #'
 #' @inheritParams guide_legend
+#' @param breaks,labels,minor.breaks Either `derive()` to indicate that the
+#'   breaks, labels or minor breaks should be taken from the scale, or valid
+#'   input to the scale's synonymous arguments to override the scale's settings.
+#'   By default, `breaks` and `labels` are derived from the scale, whereas
+#'   `minor.breaks` are omitted.
+#' @param trans A `function`, `formula` or `<trans>` object that can perform
+#'   the transformation for secondary axes. Note that discrete scales cannot
+#'   be transformed. The default, `NULL`, will perform no transformation.
 #' @param check.overlap silently remove overlapping labels,
 #'   (recursively) prioritizing the first, last, and middle labels.
 #' @param angle Compared to setting the angle in [theme()] / [element_text()],
@@ -41,9 +49,19 @@
 #'
 #' # can also be used to add a duplicate guide
 #' p + guides(x = guide_axis(n.dodge = 2), y.sec = guide_axis())
-guide_axis <- function(title = waiver(), check.overlap = FALSE, angle = NULL,
-                       n.dodge = 1, cap = "none", order = 0,
-                       position = waiver()) {
+guide_axis <- function(
+  title = waiver(),
+  breaks = derive(),
+  labels = derive(),
+  minor.breaks = NULL,
+  trans = NULL,
+  check.overlap = FALSE,
+  angle = NULL,
+  n.dodge = 1,
+  cap = "none",
+  order = 0,
+  position = waiver()
+) {
 
   if (is.logical(cap)) {
     check_bool(cap)
diff --git a/man/guide_axis.Rd b/man/guide_axis.Rd
index 086ba0b25a..cf90988581 100644
--- a/man/guide_axis.Rd
+++ b/man/guide_axis.Rd
@@ -6,6 +6,10 @@
 \usage{
 guide_axis(
   title = waiver(),
+  breaks = derive(),
+  labels = derive(),
+  minor.breaks = NULL,
+  trans = NULL,
   check.overlap = FALSE,
   angle = NULL,
   n.dodge = 1,
@@ -20,6 +24,16 @@ If \code{NULL}, the title is not shown. By default
 (\code{\link[=waiver]{waiver()}}), the name of the scale object or the name
 specified in \code{\link[=labs]{labs()}} is used for the title.}
 
+\item{breaks, labels, minor.breaks}{Either \code{derive()} to indicate that the
+breaks, labels or minor breaks should be taken from the scale, or valid
+input to the scale's synonymous arguments to override the scale's settings.
+By default, \code{breaks} and \code{labels} are derived from the scale, whereas
+\code{minor.breaks} are omitted.}
+
+\item{trans}{A \code{function}, \code{formula} or \verb{<trans>} object that can perform
+the transformation for secondary axes. Note that discrete scales cannot
+be transformed. The default, \code{NULL}, will perform no transformation.}
+
 \item{check.overlap}{silently remove overlapping labels,
 (recursively) prioritizing the first, last, and middle labels.}
 

From db6647885e020d3ba45a513a1b55d553990577c6 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 15:00:58 +0200
Subject: [PATCH 05/13] Move discrete scale warning

---
 R/guide-axis.R | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index e5393dde13..d40b62c5d0 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -136,10 +136,6 @@ GuideAxis <- ggproto(
 
     # Resolve transformation
     trans  <- function_as_trans(trans, limits, scale$scale$trans)
-    if (!is.null(trans) && scale$is_discrete()) {
-      cli::cli_warn("Cannot use axis transformation with discrete scales.")
-      trans <- NULL
-    }
 
     if (!is.null(trans) || !is.derived(breaks) || !is.derived(labels)) {
       # If anything needs to be computed that is not included in the viewscale,
@@ -625,9 +621,16 @@ axis_label_element_overrides <- function(axis_position, angle = NULL) {
 }
 
 function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
+  if (is.null(fun)) {
+    return(NULL)
+  }
   if (is.character(fun)) {
     fun <- as.trans(fun)
   }
+  if (!is.null(fun) && !is.numeric(limits)) {
+    cli::cli_warn("Cannot use axis transformation with discrete scales.")
+    return(NULL)
+  }
   if (is.trans(fun)) {
     if (fun$name == "identity") {
       return(NULL)

From bf8b85740cb17ba1a4385d85272b6aa5a5d60e2f Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 15:29:58 +0200
Subject: [PATCH 06/13] More transformation error-handling

---
 R/guide-axis.R | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index d40b62c5d0..1f4339aaf8 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -68,7 +68,7 @@ guide_axis <- function(
     cap <- if (cap) "both" else "none"
   }
   cap <- arg_match0(cap, c("none", "both", "upper", "lower"))
-
+  check_breaks_labels(breaks, labels)
 
   new_guide(
     title = title,
@@ -652,6 +652,23 @@ function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
   origin_seq <- scale_trans$inverse(limits_seq)
   trans_seq  <- fun(origin_seq)
 
+  if (length(trans_seq) != detail) {
+    cli::cli_abort(
+      "The {.arg trans} transformation must preserve the length of input."
+    )
+  }
+
+  finite <- is.finite(trans_seq)
+  origin_seq <- origin_seq[finite]
+  trans_seq  <- trans_seq[finite]
+
+  if (length(trans_seq) < detail / 100) {
+    cli::cli_abort(paste0(
+      "The {.arg trans} transformation could not transform the range ",
+      "{.field [{limits[1]}, {limits[2]}]}."
+    ))
+  }
+
   # Test for monotonicity
   if (!is_unique(sign(diff(trans_seq)))) {
     cli::cli_abort("The {.arg trans} transformation must be monotonic.")

From 97b4b2f80b4c1a805e122f4fc3a07dd030672557 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 15:48:44 +0200
Subject: [PATCH 07/13] Tweaks to limits

---
 R/guide-axis.R | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 1f4339aaf8..665697db29 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -132,24 +132,28 @@ GuideAxis <- ggproto(
   ),
 
   extract_key = function(scale, aesthetic, breaks, labels, trans, ...) {
-    limits <- scale$continuous_range
 
-    # Resolve transformation
-    trans  <- function_as_trans(trans, limits, scale$scale$trans)
+    # Retrieve limits information
+    limits <- scale$get_limits()
+    range  <- scale$continuous_range
+
+    # Resolve transformations
+    scale_trans <- scale$scale$trans %||% identity_trans()
+    trans  <- function_as_trans(trans, range, scale_trans)
 
     if (!is.null(trans) || !is.derived(breaks) || !is.derived(labels)) {
       # If anything needs to be computed that is not included in the viewscale,
       # a temporary scale computes the necessary components
       temp_scale <- ggproto(
         NULL, scale$scale,
-        trans  = trans %||% scale$scale$trans,
-        limits = if (scale$is_discrete()) scale$get_limits() else limits,
         breaks = if (is.derived(breaks))  scale$scale$breaks else breaks,
         labels = if (is.derived(labels))  scale$scale$labels else labels
+        trans  = trans %||% scale_trans,
+        limits = scale_trans$inverse(limits),
       )
       # Allow plain numeric breaks for discrete scales
       if (!(scale$is_discrete() && is.numeric(breaks))) {
-        breaks <- temp_scale$get_breaks()
+        breaks <- temp_scale$get_breaks(scale_trans$inverse(range))
       }
     } else {
       temp_scale <- NULL

From 0ab0732d9ee4d1dce92cd9a96bc71a6b20f72d4c Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 15:49:07 +0200
Subject: [PATCH 08/13] Tweak to breaks

---
 R/guide-axis.R | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 665697db29..7301c96660 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -142,14 +142,20 @@ GuideAxis <- ggproto(
     trans  <- function_as_trans(trans, range, scale_trans)
 
     if (!is.null(trans) || !is.derived(breaks) || !is.derived(labels)) {
+      if (is.derived(breaks)) {
+        breaks <- scale$scale$breaks
+        if (is.waive(breaks)) {
+          breaks <- scale_trans$breaks
+        }
+      }
       # If anything needs to be computed that is not included in the viewscale,
       # a temporary scale computes the necessary components
       temp_scale <- ggproto(
         NULL, scale$scale,
-        breaks = if (is.derived(breaks))  scale$scale$breaks else breaks,
-        labels = if (is.derived(labels))  scale$scale$labels else labels
         trans  = trans %||% scale_trans,
         limits = scale_trans$inverse(limits),
+        breaks = breaks,
+        labels = if (is.derived(labels)) scale$scale$labels else labels
       )
       # Allow plain numeric breaks for discrete scales
       if (!(scale$is_discrete() && is.numeric(breaks))) {
@@ -163,8 +169,11 @@ GuideAxis <- ggproto(
     if (length(breaks) == 0) {
       return(NULL)
     }
-
-    mapped <- scale$map(breaks)
+    if (is.null(trans)) {
+      mapped <- scale$map(breaks)
+    } else {
+      mapped <- scale$map(scale_trans$transform(breaks))
+    }
 
     if (!is.null(temp_scale)) {
       labels <- temp_scale$get_labels(breaks)
@@ -692,7 +701,8 @@ function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
     "secondary_transformation",
     transform = function(x) approx(trans_seq, origin_seq, x)$y,
     inverse   = fun,
-    format    = format_format(digits = 3)
+    format    = format_format(digits = 3),
+    domain    = range(trans_seq)
   )
 }
 

From 20d490e94825fe46594e746362e9817254a709aa Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 16:08:35 +0200
Subject: [PATCH 09/13] More nuance for discrete scales

---
 R/guide-axis.R | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 7301c96660..5914690368 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -148,21 +148,28 @@ GuideAxis <- ggproto(
           breaks <- scale_trans$breaks
         }
       }
+      if (!scale$is_discrete()) {
+        limits <- scale_trans$inverse(limits)
+      }
       # If anything needs to be computed that is not included in the viewscale,
       # a temporary scale computes the necessary components
       temp_scale <- ggproto(
         NULL, scale$scale,
         trans  = trans %||% scale_trans,
-        limits = scale_trans$inverse(limits),
+        limits = limits,
         breaks = breaks,
         labels = if (is.derived(labels)) scale$scale$labels else labels
       )
       # Allow plain numeric breaks for discrete scales
-      if (!(scale$is_discrete() && is.numeric(breaks))) {
+      if (scale$is_discrete()) {
+        if (!is.numeric(breaks)) {
+          breaks <- temp_scale$get_breaks(limits)
+        }
+      } else {
         breaks <- temp_scale$get_breaks(scale_trans$inverse(range))
       }
     } else {
-      temp_scale <- NULL
+      temp_scale <- scale
       breaks <- scale$get_breaks()
     }
 
@@ -175,11 +182,7 @@ GuideAxis <- ggproto(
       mapped <- scale$map(scale_trans$transform(breaks))
     }
 
-    if (!is.null(temp_scale)) {
-      labels <- temp_scale$get_labels(breaks)
-    } else {
-      labels <- scale$get_labels(breaks)
-    }
+    labels <- temp_scale$get_labels(breaks)
     if (is.expression(labels)) {
       labels <- as.list(labels)
     }

From e510407034a6c6626017b8cc42c596461c9abaa5 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 22 Sep 2023 16:08:55 +0200
Subject: [PATCH 10/13] Add tests

---
 tests/testthat/_snaps/guides.md | 20 ++++++++
 tests/testthat/test-guides.R    | 89 +++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+)

diff --git a/tests/testthat/_snaps/guides.md b/tests/testthat/_snaps/guides.md
index 6fb109ecbd..c2c932f9d1 100644
--- a/tests/testthat/_snaps/guides.md
+++ b/tests/testthat/_snaps/guides.md
@@ -65,6 +65,26 @@
     Breaks are not formatted correctly for a bin legend.
     i Use `(<lower>, <upper>]` format to indicate bins.
 
+# guide_axis(trans) works as expected.
+
+    The `trans` argument must be a <trans> object, a formula or function, not the number 10.
+
+---
+
+    The `trans` transformation must be monotonic.
+
+---
+
+    The `trans` transformation could not transform the range [0, 10].
+
+---
+
+    The `trans` transformation must preserve the length of input.
+
+---
+
+    `breaks` and `labels` must have the same length
+
 # binning scales understand the different combinations of limits, breaks, labels, and show.limits
 
     `show.limits` is ignored when `labels` are given as a character vector.
diff --git a/tests/testthat/test-guides.R b/tests/testthat/test-guides.R
index ac704fdf79..58fce5b7e0 100644
--- a/tests/testthat/test-guides.R
+++ b/tests/testthat/test-guides.R
@@ -339,6 +339,95 @@ test_that("guide_colourbar warns about discrete scales", {
 
 })
 
+test_that("guide_axis(trans) works as expected.", {
+
+  axis_key <- function(limits = c(0, 10), ..., scale = scale_x_continuous()) {
+    scale <- scale$clone()
+    scale$train(limits)
+    vs <- view_scale_primary(scale)
+
+    guide  <- guide_axis(...)
+    params <- guide$params
+    params$position <- "bottom"
+    guide$train(params, vs, "x")$key
+  }
+
+  # Input checking
+  expect_snapshot_error(axis_key(trans = 10))
+  expect_snapshot_error(axis_key(trans = sin))
+  expect_snapshot_error(axis_key(trans = function(x) rep(Inf, length(x))))
+  expect_snapshot_error(axis_key(trans = ~ 1))
+  expect_snapshot_error(axis_key(breaks = 1:2, labels = "A"))
+  expect_silent(axis_key(trans = ~ sin(.x / 100)))
+
+  fmt <- format_format(digits = 3)
+
+  # Identity transformation
+  scale <- scale_x_continuous()
+  key <- axis_key(trans = ~., breaks = waiver(), scale = scale)
+  expect_equal(key$.label, fmt(seq(0, 10, by = 2.5)))
+  expect_equal(key$x, seq(0, 10, by = 2.5), tolerance = 1e-3)
+
+  # Works with subtraction
+  key <- axis_key(trans = ~1-., breaks = waiver(), scale = scale)
+  expect_equal(key$.label, fmt(seq(-7.5, 0, by = 2.5)))
+  expect_equal(key$x, seq(8.5, 1, by = -2.5), tolerance = 1e-3)
+
+  # Works with division
+  key <- axis_key(limits = c(1, 9), trans = ~ 10/., breaks = waiver(), scale = scale)
+  expect_equal(key$.label, fmt(10 / c(4, 2, 4/3, 1)))
+  expect_equal(key$x, c(4, 2, 4/3, 1), tolerance = 1e-3)
+
+  # Works with log transformed scales
+  ## Identity transformation
+  key <- axis_key(trans = ~.x, scale = scale_x_log10())
+  expect_equal(key$.label, fmt(10^c(0, 3, 6, 9)))
+  expect_equal(key$x, c(0, 3, 6, 9), tolerance = 1e-3)
+
+  ## Proper transformation
+  key <- axis_key(trans = ~ . * 100, scale = scale_x_log10())
+  expect_equal(key$.label, fmt(10^c(2, 5, 8, 11)))
+  expect_equal(key$x, c(0, 3, 6, 9), tolerance = 1e-3)
+
+  # Custom breaks/labels
+  key <- axis_key(
+    trans = ~ . * 100,
+    breaks = 10^seq(2, 10, by = 2),
+    labels = math_format(format = log10),
+    scale = scale_x_log10()
+  )
+  expect_equal(as.character(key$.label), paste0("10^", seq(2, 10, by = 2)))
+  expect_equal(key$x, seq(0, 8, by = 2))
+
+  # Plain discrete scale
+  key <- axis_key(
+    c("A", "B", "C"),
+    scale = scale_x_discrete()
+  )
+  expect_equal(key$.label, c("A", "B", "C"))
+  expect_equal(unclass(key$x), c(1, 2, 3))
+
+  # Discrete custom breaks/labels
+  key <- axis_key(
+    c("A", "B", "C"),
+    breaks = c("A", "C"),
+    labels = c("X", "Y"),
+    scale  = scale_x_discrete()
+  )
+  expect_equal(key$.label, c("X", "Y"))
+  expect_equal(unclass(key$x), c(1, 3))
+
+  # Discrete numeric breaks
+  key <- axis_key(
+    c("A", "B", "C"),
+    breaks = c(1.5, 2.5),
+    labels = c("foo", "bar"),
+    scale  = scale_x_discrete()
+  )
+  expect_equal(key$.label, c("foo", "bar"))
+  expect_equal(unclass(key$x), c(1.5, 2.5))
+})
+
 # Visual tests ------------------------------------------------------------
 
 test_that("axis guides are drawn correctly", {

From bee49dea423d1fe0fe777871e6751d35b7496e17 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 26 Oct 2023 09:38:52 +0200
Subject: [PATCH 11/13] Change test minor.ticks syntax

---
 tests/testthat/test-guides.R | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tests/testthat/test-guides.R b/tests/testthat/test-guides.R
index d3984903c0..5f03d2ea12 100644
--- a/tests/testthat/test-guides.R
+++ b/tests/testthat/test-guides.R
@@ -598,13 +598,13 @@ test_that("guide_axis() draws minor ticks correctly", {
     scale_x_continuous(labels = math_format()) +
     guides(
       # Test for styling and style inheritance
-      x = guide_axis(minor.ticks = TRUE),
+      x = guide_axis(minor.breaks = TRUE),
       # # Test for opposed lengths
-      y = guide_axis(minor.ticks = TRUE),
+      y = guide_axis(minor.breaks = TRUE),
       # # Test for flipped lenghts
-      x.sec = guide_axis(minor.ticks = TRUE),
+      x.sec = guide_axis(minor.breaks = TRUE),
       # # Test that minor.length doesn't influence spacing when no minor ticks are drawn
-      y.sec = guide_axis(minor.ticks = FALSE)
+      y.sec = guide_axis(minor.breaks = FALSE)
     )
   expect_doppelganger("guides with minor ticks", p)
 })

From 862f1c0c3c8270e592496bcd396d0b4b4af34be9 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Thu, 26 Oct 2023 10:11:22 +0200
Subject: [PATCH 12/13] Additional tweaks

---
 R/guide-axis.R | 62 +++++++++++++++++++++++++++-----------------------
 1 file changed, 34 insertions(+), 28 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 7643374b25..55208b520c 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -154,16 +154,14 @@ GuideAxis <- ggproto(
         is.custom(minor_breaks)) {
       if (is.derived(breaks)) {
         breaks <- scale$scale$breaks
-        if (is.waive(breaks)) {
+        if (is.waive(breaks) && !scale$is_discrete()) {
           breaks <- scale_trans$breaks
         }
       }
       if (is.derived(minor_breaks)) {
         minor_breaks <- scale$scale$minor_breaks
       }
-      if (!scale$is_discrete()) {
-        limits <- scale_trans$inverse(limits)
-      }
+      limits <- scale_trans$inverse(limits)
       # If anything needs to be computed that is not included in the viewscale,
       # a temporary scale computes the necessary components
       temp_scale <- ggproto(
@@ -174,59 +172,64 @@ GuideAxis <- ggproto(
         minor_breaks = minor_breaks,
         labels = if (is.derived(labels)) scale$scale$labels else labels
       )
-      # Allow plain numeric breaks for discrete scales
       if (scale$is_discrete()) {
+        # Allow plain numeric breaks for discrete scales
         if (!is.numeric(breaks)) {
           breaks <- temp_scale$get_breaks(limits)
         }
+        # Allow minor breaks to be a function
+        if (is.function(minor_breaks)) {
+          minor_breaks <- minor_breaks(limits)
+        }
       } else {
         breaks <- temp_scale$get_breaks(limits)
         minor_breaks <- temp_scale$get_breaks_minor(b = breaks, limits = limits)
       }
     } else {
       temp_scale <- scale
-      breaks <- scale$get_breaks()
+      if (!is.null(breaks)) {
+        breaks <- scale$get_breaks()
+      }
       if (!is.null(minor_breaks)) {
         minor_breaks <- scale$get_breaks_minor()
       }
     }
 
-    if (length(breaks) == 0) {
-      return(NULL)
-    }
     if (is.null(trans)) {
-      mapped <- scale$map(breaks)
+      map <- function(x) scale$map(x)
     } else {
-      mapped <- scale$map(scale_trans$transform(breaks))
+      map <- function(x) scale$map(scale_trans$transform(x))
     }
 
-    labels <- temp_scale$get_labels(breaks)
-    if (is.expression(labels)) {
-      labels <- as.list(labels)
+    if (!is.null(breaks)) {
+      if (!is.null(labels)) {
+        labels <- temp_scale$get_labels(breaks)
+      }
+      if (is.expression(labels)) {
+        labels <- as.list(labels)
+      }
+      key <- data_frame(!!aesthetic := map(breaks))
+      key$.value <- breaks
+      key$.label <- labels
+      key <- vec_slice(key, is_finite(breaks))
+    } else {
+      key <- data_frame0()
     }
 
-    key <- data_frame(!!aesthetic := mapped)
-    key$.value <- breaks
-    key$.label <- labels
-
-    if (is.numeric(breaks)) {
-      key <- vec_slice(key, is.finite(breaks))
-    }
     if (!is.null(minor_breaks)) {
 
       minor_breaks <- setdiff(minor_breaks, key$.value)
-      minor_breaks <- minor_breaks[is.finite(minor_breaks)]
+      minor_breaks <- minor_breaks[is_finite(minor_breaks)]
 
       if (length(minor_breaks) < 1) {
         return(key)
       }
-      if (is.null(trans)) {
-        minor <- scale$map(minor_breaks)
-      } else {
-        minor <- scale$map(scale_trans$transform(minor_breaks))
+      minor <- data_frame0(!!aesthetic := map(minor_breaks))
+
+      if (!scale$is_discrete()) {
+        minor$.value <- minor_breaks
       }
-      minor <- data_frame0(!!aesthetic := minor)
-      minor$.value <- minor_breaks
+
       minor$.type <- "minor"
 
       if (nrow(key) > 0) {
@@ -236,6 +239,9 @@ GuideAxis <- ggproto(
         return(minor)
       }
     }
+    if (nrow(key) == 0) {
+      return(NULL)
+    }
     key
   },
 

From b0fb67b8347be3ad889fde36390c862fb5cbb35d Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Fri, 27 Oct 2023 11:20:39 +0200
Subject: [PATCH 13/13] Resolve mismatch

---
 R/guide-axis.R    | 2 +-
 man/guide_axis.Rd | 4 ----
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/R/guide-axis.R b/R/guide-axis.R
index 55208b520c..a749abae6f 100644
--- a/R/guide-axis.R
+++ b/R/guide-axis.R
@@ -771,7 +771,7 @@ function_as_trans <- function(fun, limits, scale_trans, detail = 1000) {
 
   trans_new(
     "secondary_transformation",
-    transform = function(x) approx(trans_seq, origin_seq, x)$y,
+    transform = function(x) stats::approx(trans_seq, origin_seq, x)$y,
     inverse   = fun,
     format    = format_format(digits = 3),
     domain    = range(trans_seq)
diff --git a/man/guide_axis.Rd b/man/guide_axis.Rd
index 21c7b43a53..cf90988581 100644
--- a/man/guide_axis.Rd
+++ b/man/guide_axis.Rd
@@ -13,7 +13,6 @@ guide_axis(
   check.overlap = FALSE,
   angle = NULL,
   n.dodge = 1,
-  minor.ticks = FALSE,
   cap = "none",
   order = 0,
   position = waiver()
@@ -46,9 +45,6 @@ you probably want.}
 horizontal axes) that should be used to render the labels. This is
 useful for displaying labels that would otherwise overlap.}
 
-\item{minor.ticks}{Whether to draw the minor ticks (\code{TRUE}) or not draw
-minor ticks (\code{FALSE}, default).}
-
 \item{cap}{A \code{character} to cut the axis line back to the last breaks. Can
 be \code{"none"} (default) to draw the axis line along the whole panel, or
 \code{"upper"} and \code{"lower"} to draw the axis to the upper or lower break, or