Better limits + dimensions defaults for scale_discrete.

Fixes tidyverse#1542

Author: hadley

NEWS.md

@@ -73,6 +73,9 @@
 
 * The default scale for columns of class "AsIs" is now "identity" (#1518).
 
+* `scale_*_discrete()` has better defaults when used with purely continuous
+  data (#1542).
+
 * `scale_size()` warns when used with categorical data.
 
 * `scale_size()`, `scale_colour()`, and `scale_fill()` gain date and date-time

R/scale-discrete-.r

@@ -92,14 +92,9 @@ ScaleDiscretePosition <- ggproto("ScaleDiscretePosition", ScaleDiscrete,
     }
   },
 
-  # If range not available from discrete range, implies discrete scale been
-  # used with purely continuous data, so construct limits accordingly
   get_limits = function(self) {
     if (self$is_empty()) return(c(0, 1))
-
-    dis_limits <- function(x) seq.int(floor(min(x)), ceiling(max(x)), by = 1L)
-
-    self$limits %||% self$range$range %||% dis_limits(self$range_c$range)
+    self$limits %||% self$range$range %||% integer()
   },
 
   is_empty = function(self) {
@@ -120,16 +115,21 @@ ScaleDiscretePosition <- ggproto("ScaleDiscretePosition", ScaleDiscrete,
   },
 
   dimension = function(self, expand = c(0, 0)) {
-    disc_range <- c(1, length(self$get_limits()))
-    disc <- expand_range(disc_range, 0, expand[2], 1)
-
-    # if no data was trained (i.e. range_c is infinite) return disc range
-    if (any(is.infinite(self$range_c$range))) {
-      return(disc)
+    c_range <- self$range_c$range
+    d_range <- self$range$range
+
+    if (self$is_empty()) {
+      c(0, 1)
+    } else if (is.null(d_range)) { # only continuous
+      expand_range(c_range, expand[1], 0 , 1)
+    } else if (is.null(c_range)) { # only discrete
+      expand_range(c(1, length(d_range)), 0, expand[2], 1)
+    } else { # both
+      range(
+        expand_range(c_range, expand[1], 0 , 1),
+        expand_range(c(1, length(d_range)), 0, expand[2], 1)
+      )
     }
-
-    cont <- expand_range(self$range_c$range, expand[1], 0, expand[2])
-    range(disc, cont)
   },
 
   clone = function(self) {

tests/testthat/test-scale-discrete.R

@@ -0,0 +1,18 @@
+context("scale_discrete")
+
+# Ranges ------------------------------------------------------------------
+
+test_that("discrete ranges also encompas continuous values", {
+  df <- data.frame(x1 = c("a", "b", "c"), x2 = c(0, 2, 4), y = 1:3)
+
+  base <- ggplot(df, aes(y = y)) + scale_x_discrete()
+
+  x_range <- function(x) {
+    layer_scales(x)$x$dimension()
+  }
+
+  expect_equal(x_range(base + geom_point(aes(x1))), c(1, 3))
+  expect_equal(x_range(base + geom_point(aes(x2))), c(0, 4))
+  expect_equal(x_range(base + geom_point(aes(x1)) + geom_point(aes(x2))), c(0, 4))
+})
+