stat_bin() accepts functions for argument breaks (#5963)

* `stat_bin()` accepts functions for argument `breaks`

* add news bullet

Author: aijordan

Diff for: NEWS.md

@@ -1,5 +1,6 @@
 # ggplot2 (development version)
 
+* `stat_bin()` now accepts functions for argument `breaks` (@aijordan, #4561)
 * (internal) The plot's layout now has a coord parameter that is used to 
   prevent setting up identical panel parameters (#5427)
 * (internal) rearranged the code of `Facet$draw_panels()` method (@teunbrand).

Diff for: R/stat-bin.R

@@ -22,7 +22,7 @@
 #'   outside the range of the data.
 #' @param breaks Alternatively, you can supply a numeric vector giving
 #'    the bin boundaries. Overrides `binwidth`, `bins`, `center`,
-#'    and `boundary`.
+#'    and `boundary`. Can also be a function that takes group-wise values as input and returns bin boundaries.
 #' @param closed One of `"right"` or `"left"` indicating whether right
 #'   or left edges of bins are included in the bin.
 #' @param pad If `TRUE`, adds empty bins at either end of x. This ensures
@@ -146,6 +146,9 @@ StatBin <- ggproto("StatBin", Stat,
                            origin = NULL, right = NULL, drop = NULL) {
     x <- flipped_names(flipped_aes)$x
     if (!is.null(breaks)) {
+      if (is.function(breaks)) {
+        breaks <- breaks(data[[x]])
+      }
       if (!scales[[x]]$is_discrete()) {
          breaks <- scales[[x]]$transform(breaks)
       }

Diff for: man/geom_histogram.Rd

@@ -60,6 +60,14 @@ test_that("can use breaks argument", {
   expect_equal(out$count, c(1, 2))
 })
 
+test_that("breaks computes bin boundaries for function input", {
+  df <- data.frame(x = c(0, 0, 0, 1:3))
+  out <- layer_data(ggplot(df, aes(x)) +
+                      geom_histogram(breaks = function(x) c(0, 0.5, 2.5, 7.5)))
+
+  expect_equal(out$count, c(3, 2, 1))
+})
+
 test_that("fuzzy breaks are used when cutting", {
   df <- data_frame(x = c(-1, -0.5, -0.4, 0))
   p <- ggplot(df, aes(x)) +