Proper number of bins in stat_contour()

NEWS.md

@@ -11,6 +11,9 @@
 * `stat_function()` now works with transformed y axes, e.g. `scale_y_log10()`
   (@clauswilke, #3905).
 
+* A bug was fixed in `stat_contour()` when calculating breaks based on 
+  the `bins` argument (@clauswilke, #3879).
+
 * A newly added geom `geom_density_2d_filled()` and associated stat 
   `stat_density_2d_filled()` can draw filled density contours
   (@clauswilke, #3846).

R/geom-contour.r

@@ -34,8 +34,8 @@
 #' v + geom_contour_filled()
 #'
 #' # Setting bins creates evenly spaced contours in the range of the data
+#' v + geom_contour(bins = 3)
 #' v + geom_contour(bins = 5)
-#' v + geom_contour(bins = 10)
 #'
 #' # Setting binwidth does the same thing, parameterised by the distance
 #' # between contours

R/stat-contour.r

@@ -151,19 +151,26 @@ contour_breaks <- function(z_range, bins = NULL, binwidth = NULL, breaks = NULL)
   # If no parameters set, use pretty bins
   if (is.null(bins) && is.null(binwidth)) {
     breaks <- pretty(z_range, 10)
+    return(breaks)
   }
 
   # If provided, use bins to calculate binwidth
   if (!is.null(bins)) {
     binwidth <- diff(z_range) / (bins - 1)
-  }
-
-  # If necessary, compute breaks from binwidth
-  if (is.null(breaks)) {
     breaks <- fullseq(z_range, binwidth)
+
+    # Sometimes the above sequence yields one bin too few.
+    # If this happens, try again.
+    if (length(breaks) < bins + 1) {
+      binwidth <- diff(z_range) / bins
+      breaks <- fullseq(z_range, binwidth)
+    }
+
+    return(breaks)
   }
 
-  breaks
+  # if we haven't returned yet, compute breaks from binwidth
+  fullseq(z_range, binwidth)
 }
 
 #' Compute isoband objects

tests/testthat/test-stat-contour.R

@@ -36,7 +36,9 @@ test_that("contour breaks can be set manually and by bins and binwidth", {
   range <- c(0, 1)
   expect_equal(contour_breaks(range), pretty(range, 10))
   expect_identical(contour_breaks(range, breaks = 1:3), 1:3)
-  expect_length(contour_breaks(range, bins = 5), 5)
+  expect_length(contour_breaks(range, bins = 5), 6)
+  # shifting the range by 0.2 hits another execution branch in contour_breaks()
+  expect_length(contour_breaks(range + 0.2, bins = 5), 6)
   expect_equal(resolution(contour_breaks(range, binwidth = 0.3)), 0.3)
 })