Allow stat_function to work with discrete x

Fixes #1509

Author: hadley

NEWS.md

@@ -2,6 +2,8 @@
 
 ## Bug fixes and minor improvements
 
+* `stat_function()` once again works with discrete x axes (#1509).
+
 * All `\donttest{}` examples run.
 
 * Removed a superfluous comma in `theme-defaults.r` code (@jschoeley)

R/stat-function.r

@@ -80,9 +80,17 @@ StatFunction <- ggproto("StatFunction", Stat,
     range <- scales$x$dimension()
     xseq <- seq(range[1], range[2], length.out = n)
 
+    if (scales$x$is_discrete()) {
+      x_trans <- xseq
+    } else {
+      # For continuous scales, need to back transform from transformed range
+      # to original values
+      x_trans <- scales$x$trans$inverse(xseq)
+    }
+
     data.frame(
       x = xseq,
-      y = do.call(fun, c(list(quote(scales$x$trans$inverse(xseq))), args))
+      y = do.call(fun, c(list(quote(x_trans)), args))
     )
   }
 )

tests/testthat/test-stats-function.r

@@ -23,3 +23,14 @@ test_that("uses scale limits, not data limits", {
   expect_false(any(is.na(ret$y)))
   expect_false(any(is.na(ret_log$y)))
 })
+
+test_that("works with discrete x", {
+  dat <- data.frame(x = c("a", "b"))
+
+  base <- ggplot(dat, aes(x, group = 1)) +
+    stat_function(fun = as.numeric, geom = "point", n = 2)
+  ret <- layer_data(base)
+
+  expect_equal(ret$x, 1:2)
+  expect_equal(ret$y, 1:2)
+})