Make stat_function() work with empty input data. (#3984)

* make stat_function() work with empty input data. fixes #3983.

* update news item, fix documentation, add unit tests

* one more test

Author: clauswilke

NEWS.md

@@ -8,9 +8,10 @@
 
 * Default continuous color scales (i.e., the `options()` `ggplot2.continuous.colour` and `ggplot2.continuous.fill`, which inform the `type` argument of `scale_fill_continuous()` and `scale_colour_continuous()`) now accept a function, which allows more control over these default `continuous_scale()`s (@cpsievert, #3827)
 
-* A newly added `geom_function()` is now the recommended geom to use in
-  conjunction with `stat_function()`. In addition, `stat_function()` now
-  works with transformed y axes, e.g. `scale_y_log10()` (@clauswilke, #3611, #3905).
+* A newly added `geom_function()` is now recommended to use in conjunction
+  with/instead of `stat_function()`. In addition, `stat_function()` now
+  works with transformed y axes, e.g. `scale_y_log10()`, and in plots
+  containing no other data or layers (@clauswilke, #3611, #3905, #3983).
 
 * A bug was fixed in `stat_contour()` when calculating breaks based on 
   the `bins` argument (@clauswilke, #3879).

R/geom-function.R

@@ -18,7 +18,7 @@
 #'   geom_function(fun = dnorm, colour = "red")
 #'
 #' # To plot functions without data, specify range of x-axis
-#' base <- ggplot(data.frame(x = c(-5, 5)), aes(x))
+#' base <- ggplot() + xlim(-5, 5)
 #' base + geom_function(fun = dnorm)
 #' base + geom_function(fun = dnorm, args = list(mean = 2, sd = .5))
 #'
@@ -45,8 +45,11 @@ geom_function <- function(mapping = NULL, data = NULL, stat = "function",
                           position = "identity", ..., na.rm = FALSE,
                           show.legend = NA, inherit.aes = TRUE) {
   # Warn if supplied data is going to be overwritten
-  if (!is.null(data) && identical(stat, "function")) {
-    warn("`data` is not used by stat_function()")
+  if (identical(stat, "function")) {
+    if (!is.null(data)) {
+      warn("`data` is not used by stat_function()")
+    }
+    data <- ensure_nonempty_data
   }
 
   layer(

R/stat-function.r

@@ -29,6 +29,7 @@ stat_function <- function(mapping = NULL, data = NULL,
   if (!is.null(data)) {
     warn("`data` is not used by stat_function()")
   }
+  data <- ensure_nonempty_data
 
   layer(
     data = data,
@@ -57,15 +58,21 @@ StatFunction <- ggproto("StatFunction", Stat,
   default_aes = aes(y = after_scale(y)),
 
   compute_group = function(data, scales, fun, xlim = NULL, n = 101, args = list()) {
-    range <- xlim %||% scales$x$dimension()
-    xseq <- seq(range[1], range[2], length.out = n)
-
-    if (scales$x$is_discrete()) {
+    if (is.null(scales$x)) {
+      range <- xlim %||% c(0, 1)
+      xseq <- seq(range[1], range[2], length.out = n)
       x_trans <- xseq
     } else {
-      # For continuous scales, need to back transform from transformed range
-      # to original values
-      x_trans <- scales$x$trans$inverse(xseq)
+      range <- xlim %||% scales$x$dimension()
+      xseq <- seq(range[1], range[2], length.out = n)
+
+      if (scales$x$is_discrete()) {
+        x_trans <- xseq
+      } else {
+        # For continuous scales, need to back transform from transformed range
+        # to original values
+        x_trans <- scales$x$trans$inverse(xseq)
+      }
     }
 
     if (is.formula(fun)) fun <- as_function(fun)
@@ -82,3 +89,15 @@ StatFunction <- ggproto("StatFunction", Stat,
     ))
   }
 )
+
+# Convenience function used by `stat_function()` and
+# `geom_function()` to convert empty input data into
+# non-empty input data without touching any non-empty
+# input data that may have been provided.
+ensure_nonempty_data <- function(data) {
+  if (empty(data)) {
+    new_data_frame(list(group = 1), n = 1)
+  } else {
+    data
+  }
+}

man/geom_function.Rd

@@ -24,6 +24,37 @@ test_that("uses scale limits, not data limits", {
   expect_false(any(is.na(ret_log$y)))
 })
 
+test_that("works in plots without any data", {
+  f <- function(x) 2*x
+
+  # default limits, 0 to 1
+  base <- ggplot() + geom_function(fun = f, n = 6)
+  ret <- layer_data(base)
+  expect_identical(ret$x, seq(0, 1, length.out = 6))
+  expect_identical(ret$y, 2*ret$x)
+
+  # manually set limits with xlim()
+  base <- ggplot() + xlim(0, 2) + geom_function(fun = f, n = 6)
+  ret <- layer_data(base)
+  expect_identical(ret$x, seq(0, 2, length.out = 6))
+  expect_identical(ret$y, 2*ret$x)
+
+  # manually set limits with xlim argument
+  base <- ggplot() + geom_function(fun = f, n = 6, xlim = c(0, 2))
+  ret <- layer_data(base)
+  expect_identical(ret$x, seq(0, 2, length.out = 6))
+  expect_identical(ret$y, 2*ret$x)
+
+  # mapping of color via aes() works
+  base <- ggplot() +
+    geom_function(aes(color = "fun"), fun = f, n = 6) +
+    scale_color_manual(values = c(fun = "#D55E00"))
+  ret <- layer_data(base)
+  expect_identical(ret$x, seq(0, 1, length.out = 6))
+  expect_identical(ret$y, 2*ret$x)
+  expect_identical(ret$colour, rep("#D55E00", 6))
+})
+
 test_that("works with discrete x", {
   dat <- data_frame(x = c("a", "b"))