Fix the discrete scale when all values are continuous (#3487)

Author: yutannihilation

R/scale-discrete-.r

@@ -83,17 +83,18 @@ ScaleDiscretePosition <- ggproto("ScaleDiscretePosition", ScaleDiscrete,
   },
 
   get_limits = function(self) {
-      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$range)
-      } else {
-          integer(0)
-      }
+    # if scale contains no information, return the default limit
+    if (self$is_empty()) {
+      return(c(0, 1))
+    }
+
+    # if self$limits is not NULL and is a function, apply it to range
+    if (is.function(self$limits)){
+      return(self$limits(self$range$range))
+    }
+
+    # self$range$range can be NULL because non-discrete values use self$range_c
+    self$limits %||% self$range$range %||% integer()
   },
 
   is_empty = function(self) {

tests/testthat/test-scale-discrete.R

@@ -62,6 +62,12 @@ test_that("discrete ranges also encompass continuous values", {
   expect_equal(x_range(base + geom_point(aes(x1)) + geom_point(aes(x2))), c(0, 4))
 })
 
+test_that("discrete ranges have limits even when all values are continuous", {
+  scale <- scale_x_discrete()
+  scale$train(1:3)
+  expect_identical(scale$get_limits(), integer())
+})
+
 test_that("discrete scale shrinks to range when setting limits", {
   df <- data_frame(x = letters[1:10], y = 1:10)
   p <- ggplot(df, aes(x, y)) + geom_point() +