Discrete labels from break names

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* When discrete breaks have names, they'll be used as labels by default 
+  (@teunbrand, #6147).
 * Fixed bug where the `ggplot2::`-prefix did not work with `stage()` 
   (@teunbrand, #6104).
 * New `get_labs()` function for retrieving completed plot labels 

R/axis-secondary.R

@@ -186,7 +186,7 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
     if (is.derived(self$breaks)) self$breaks <- scale$breaks
     if (is.waive(self$breaks)) {
       if (scale$is_discrete()) {
-        self$breaks <- scale$get_breaks()
+        self$breaks <- setNames(nm = scale$get_breaks())
       } else {
         breaks <- scale$get_transformation()$breaks
         n_breaks <- scale$n.breaks
@@ -235,7 +235,7 @@ AxisSecondary <- ggproto("AxisSecondary", NULL,
       self$mono_test(scale)
       breaks <- self$breaks
     } else {
-      breaks <- scale$map(self$breaks)
+      breaks <- setNames(scale$map(self$breaks), names(self$breaks))
     }
 
     # Get scale's original range before transformation

R/scale-.R

@@ -1029,7 +1029,8 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
     }
 
     # Breaks only occur only on values in domain
-    in_domain <- intersect(breaks, limits)
+    breaks <- setNames(as.character(breaks), names(breaks))
+    in_domain <- vec_set_intersect(breaks, as.character(limits))
     structure(in_domain, pos = match(in_domain, breaks))
   },
 
@@ -1083,6 +1084,9 @@ ScaleDiscrete <- ggproto("ScaleDiscrete", Scale,
     }
 
     if (is.waive(self$labels)) {
+      if (!is.null(names(breaks))) {
+        return(names(breaks))
+      }
       if (is.numeric(breaks)) {
         # Only format numbers, because on Windows, format messes up encoding
         format(breaks, justify = "none")

tests/testthat/test-scales-breaks-labels.R

@@ -273,6 +273,17 @@ test_that("equal length breaks and labels can be passed to ViewScales with limit
   expect_identical(test_view_scale_rev$get_labels(), c(c("0", "20", "40")))
 })
 
+test_that("break names are returned as labels", {
+
+  sc <- scale_x_continuous(breaks = c(A = 10, B = 20, C = 30))
+  sc$train(c(10, 30))
+  expect_equal(sc$get_labels(), c("A", "B", "C"))
+
+  sc <- scale_x_discrete(breaks = c(foo = "A", bar = "B", qux = "C"))
+  sc$train(c(LETTERS[1:3]))
+  expect_equal(sc$get_labels(), c("foo", "bar", "qux"))
+})
+
 # Visual tests ------------------------------------------------------------
 
 test_that("minor breaks draw correctly", {