Adhere more strictly to bins argument in stat_bin() (#5891)

* try respecting `bins` better

* fix #5890

* add test

* add news bullet

Author: teunbrand

NEWS.md

@@ -83,6 +83,8 @@
   `geom_bar()` (#3142).
 * Fix a bug in `position_jitterdodge()` where different jitters would be applied
   to different position aesthetics of the same axis (@teunbrand, #5818).
+* In `stat_bin()`, the default `boundary` is now chosen to better adhere to 
+  the `nbin` argument (@teunbrand, #5882, #5036)
 
 # ggplot2 3.5.1
 

R/bin.R

@@ -117,8 +117,12 @@ bin_breaks_bins <- function(x_range, bins = 30, center = NULL,
   } else if (bins == 1) {
     width <- diff(x_range)
     boundary <- x_range[1]
+    center <- NULL
   } else {
     width <- (x_range[2] - x_range[1]) / (bins - 1)
+    if (is.null(center)) {
+      boundary <- boundary %||% x_range[1] - width / 2
+    }
   }
 
   bin_breaks_width(x_range, width, boundary = boundary, center = center,

tests/testthat/test-stat-bin.R

@@ -119,6 +119,30 @@ test_that("bins() computes fuzz with non-finite breaks", {
   expect_equal(difference[2], 1000 * .Machine$double.eps, tolerance = 0)
 })
 
+test_that("bins is strictly adhered to", {
+
+  nbins <- c(1, 2, 3, 4, 5, 10, 20, 30, 40, 50)
+
+  # Default case
+  nbreaks <- vapply(nbins, function(bins) {
+    length(bin_breaks_bins(c(0, 10), bins)$breaks)
+  }, numeric(1))
+  expect_equal(nbreaks, nbins + 1)
+
+  # Center is provided
+  nbreaks <- vapply(nbins, function(bins) {
+    length(bin_breaks_bins(c(0, 10), bins, center = 0)$breaks)
+  }, numeric(1))
+  expect_equal(nbreaks, nbins + 1)
+
+  # Boundary is provided
+  nbreaks <- vapply(nbins, function(bins) {
+    length(bin_breaks_bins(c(0, 10), bins, boundary = 0)$breaks)
+  }, numeric(1))
+  expect_equal(nbreaks, nbins + 1)
+
+})
+
 comp_bin <- function(df, ...) {
   plot <- ggplot(df, aes(x = x)) + stat_bin(...)
   get_layer_data(plot)