Allowed reversing of discrete position scales (#3426, fixes #3115)

* reverse discrete position scales - get_limits

Author: AnneLyng

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Allowed reversing of discrete scales by re-writing `get_limits()` (@AnneLyng, #3115)
+
 * Added `stat_contour_filled()` and `geom_contour_filled()`, which compute 
   and draw filled contours of gridded data (@paleolimbot, #3044).
 

R/scale-discrete-.r

@@ -83,9 +83,17 @@ ScaleDiscretePosition <- ggproto("ScaleDiscretePosition", ScaleDiscrete,
   },
 
   get_limits = function(self) {
-    if (self$is_empty()) return(c(0, 1))
-
-    self$limits %||% self$range$range %||% integer()
+      if (self$is_empty()) {
+          c(0, 1)
+      } else if (!is.null(self$limits) & !is.function(self$limits)){
+          self$limits
+      } else if (is.null(self$limits)) {
+          self$range$range
+      } else if (is.function(self$limits)) {
+          self$limits(self$range$range)
+      } else {
+          integer(0)
+      }
   },
 
   is_empty = function(self) {

tests/testthat/test-scale-discrete.R

@@ -69,3 +69,9 @@ test_that("discrete scale shrinks to range when setting limits", {
 
   expect_equal(layer_scales(p)$x$dimension(c(0, 1)), c(0, 3))
 })
+
+test_that("discrete position scales can accept functional limits", {
+  scale <- scale_x_discrete(limits = rev)
+  scale$train(c("a", "b", "c"))
+  expect_identical(scale$get_limits(), c("c", "b", "a"))
+})