Fix off-by-one error in stat_bar.

Fixes #1487

Author: hadley

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 2.0.0.9000
 
+* `geom_histgram(bins = n)` now gives a histogram with `n` bins, not `n + 1`
+  (#1487).
+
 * `geom_tile()` once again accepts `width` and `height` parameters (#1513).
 
 * Add access to `bw` argument of `density` in `stat_density`, which makes

R/stat-bin.r

@@ -94,8 +94,14 @@ bin <- function(x, weight=NULL, binwidth=NULL, bins=NULL, origin=NULL, breaks=NU
   weight[is.na(weight)] <- 0
 
   if (is.null(range))    range    <- range(x, na.rm = TRUE, finite = TRUE)
-  if (is.null(bins))     bins     <- 30
-  if (is.null(binwidth)) binwidth <- diff(range) / bins
+
+  if (is.null(bins)) {
+    bins <- 30
+  } else {
+    stopifnot(is.numeric(bins), length(bins) == 1, bins > 1)
+  }
+
+  if (is.null(binwidth)) binwidth <- diff(range) / (bins - 1)
 
   if (is.integer(x)) {
     bins <- x

tests/testthat/test-stat-bin.R

@@ -10,6 +10,16 @@ test_that("stat_bin throws error when y aesthetic present", {
     "Unknown parameters: y")
 })
 
+test_that("bins specifies the number of bins", {
+  df <- data.frame(x = 1:100)
+  out <- function(x, ...) {
+    layer_data(ggplot(df, aes(x)) + geom_histogram(..., drop = TRUE))
+  }
+
+  expect_equal(nrow(out(bins = 2)), 2)
+  expect_equal(nrow(out(bins = 10)), 10)
+})
+
 test_that("stat_count throws error when y aesthetic present", {
   dat <- data.frame(x = c("a", "b", "c"), y = c(1, 5, 10))