Allows breaks to be a function in geom_contour()

NEWS.md

@@ -5,6 +5,8 @@
   all `values` on the legend instead, use
   `scale_*_manual(values = vals, limits = names(vals))`. (@teunbrand, @banfai, 
   #4511, #4534)
+
+* `geom_contour()` now accepts a function in the `breaks` argument (@eliocamp, #4652).
 
 # ggplot2 3.3.5
 This is a very small release focusing on fixing a couple of untenable issues 

R/geom-contour.r

@@ -19,9 +19,14 @@
 #' @inheritParams geom_path
 #' @param bins Number of contour bins. Overridden by `binwidth`.
 #' @param binwidth The width of the contour bins. Overridden by `breaks`.
-#' @param breaks Numeric vector to set the contour breaks. Overrides `binwidth`
-#'   and `bins`. By default, this is a vector of length ten with [pretty()]
-#'   breaks.
+#' @param breaks One of:
+#'   - Numeric vector to set the contour breaks
+#'   - A function that takes the range of the data and binwidth as input
+#'   and returns breaks as output. A function can be created from a formula
+#'   (e.g. ~ fullseq(.x, .y)).
+#'
+#'   Overrides `binwidth` and `bins`. By default, this is a vector of length
+#'   ten with [pretty()] breaks.
 #' @seealso [geom_density_2d()]: 2d density contours
 #' @export
 #' @examples

R/stat-contour.r

@@ -144,10 +144,17 @@ StatContourFilled <- ggproto("StatContourFilled", Stat,
 #' @noRd
 #'
 contour_breaks <- function(z_range, bins = NULL, binwidth = NULL, breaks = NULL) {
-  if (!is.null(breaks)) {
+  breaks <- allow_lambda(breaks)
+
+  if (is.numeric(breaks)) {
     return(breaks)
   }
 
+  breaks_fun <- fullseq
+  if (is.function(breaks)) {
+    breaks_fun <- breaks
+  }
+
   # If no parameters set, use pretty bins
   if (is.null(bins) && is.null(binwidth)) {
     breaks <- pretty(z_range, 10)
@@ -167,20 +174,20 @@ contour_breaks <- function(z_range, bins = NULL, binwidth = NULL, breaks = NULL)
     }
 
     binwidth <- diff(z_range) / (bins - 1)
-    breaks <- fullseq(z_range, binwidth)
+    breaks <- breaks_fun(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)
+      breaks <- breaks_fun(z_range, binwidth)
     }
 
     return(breaks)
   }
 
   # if we haven't returned yet, compute breaks from binwidth
-  fullseq(z_range, binwidth)
+  breaks_fun(z_range, binwidth)
 }
 
 #' Compute isoband objects

tests/testthat/test-stat-contour.R

@@ -32,14 +32,16 @@ test_that("contouring irregularly spaced data works", {
   expect_setequal(d8$y, c(2, 20/9, 16/9))
 })
 
-test_that("contour breaks can be set manually and by bins and binwidth", {
+test_that("contour breaks can be set manually and by bins and binwidth and a function", {
   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), 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)
+  expect_equal(contour_breaks(range), contour_breaks(range, breaks = fullseq))
+  expect_equal(contour_breaks(range), contour_breaks(range, breaks = ~fullseq(.x, .y)))
 })
 
 test_that("geom_contour_filled() and stat_contour_filled() result in identical layer data", {