stat_bin accepts functions for binwidth

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 2.2.1.9000
 
+* `stat_bin` now accepts functions for `binwidth`. This allows better binning when faceting along variables with different ranges (@botanize).
+
 * Legends no longer try and use set aesthetics that are not length one
   (fixes #1932).
 

R/geom-histogram.r

@@ -74,6 +74,13 @@
 #' m <- ggplot(movies, aes(x = rating))
 #' m + geom_histogram(binwidth = 0.5) + scale_y_sqrt()
 #' }
+#'
+#' # You can specify a function for calculating binwidth,
+#' # particularly useful when faceting along variables with
+#' # different ranges
+#' mtlong <- reshape2::melt(mtcars)
+#' ggplot(mtlong, aes(value)) + facet_wrap(~variable, scales = 'free_x') +
+#'   geom_histogram(binwidth = function(x) 2 * IQR(x) / (length(x)^(1/3)))
 geom_histogram <- function(mapping = NULL, data = NULL,
                            stat = "bin", position = "stack",
                            ...,

R/stat-bin.r

@@ -1,7 +1,9 @@
 #' \code{stat_bin} is suitable only for continuous x data. If your x data is
 #'   discrete, you probably want to use \code{\link{stat_count}}.
 #'
-#' @param binwidth The width of the bins. The default is to use \code{bins}
+#' @param binwidth The width of the bins. Can be specified as a numeric value,
+#'   or a function that calculates width from x.
+#'   The default is to use \code{bins}
 #'   bins that cover the range of the data. You should always override
 #'   this value, exploring multiple widths to find the best to illustrate the
 #'   stories in your data.
@@ -130,8 +132,11 @@ StatBin <- ggproto("StatBin", Stat,
     if (!is.null(breaks)) {
       bins <- bin_breaks(breaks, closed)
     } else if (!is.null(binwidth)) {
-      bins <- bin_breaks_width(scales$x$dimension(), binwidth, center = center,
-        boundary = boundary, closed = closed)
+      if (is.function(binwidth)) {
+        binwidth <- binwidth(data$x)
+      }
+      bins <- bin_breaks_width(scales$x$dimension(), binwidth,
+        center = center, boundary = boundary, closed = closed)
     } else {
       bins <- bin_breaks_bins(scales$x$dimension(), bins, center = center,
         boundary = boundary, closed = closed)

tests/testthat/test-stat-bin.R

@@ -22,6 +22,13 @@ test_that("bins specifies the number of bins", {
   expect_equal(nrow(out(bins = 100)), 100)
 })
 
+test_that("binwidth computes widths for function input", {
+  df <- data.frame(x = 1:100)
+  out <- layer_data(ggplot(df, aes(x)) + geom_histogram(binwidth = function(x) 5))
+
+  expect_equal(nrow(out), 21)
+})
+
 test_that("geom_histogram defaults to pad = FALSE", {
   df <- data.frame(x = 1:3)
   out <- layer_data(ggplot(df, aes(x)) + geom_histogram(binwidth = 1))