Violin resolution (#5402)

* Preserve mapped_discrete class

* Add test

* Add news bullet

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* `stat_ydensity()` with incomplete groups calculates the default `width` 
+  parameter more stably (@teunbrand, #5396)
+
 * `geom_boxplot()` gains a new argument, `staplewidth` that can draw staples
   at the ends of whiskers (@teunbrand, #5126)
 

R/stat-ydensity.R

@@ -102,11 +102,14 @@ StatYdensity <- ggproto("StatYdensity", Stat,
     )
 
     dens$y <- dens$x
-    dens$x <- mean(range(data$x))
 
     # Compute width if x has multiple values
     if (vec_unique_count(data$x) > 1) {
+      dens$x <- mean(range(data$x))
       width <- diff(range(data$x)) * 0.9
+    } else {
+      # Explicitly repeat to preserve data$x's mapped_discrete class
+      dens$x <- vec_rep(data$x[1], nrow(dens))
     }
     dens$width <- width
 

tests/testthat/test-stat-ydensity.R

@@ -25,3 +25,19 @@ test_that("`drop = FALSE` preserves groups with 1 observations", {
   )
   expect_equal(length(unique(ld$x)), 4)
 })
+
+test_that("mapped_discrete class is preserved", {
+
+  df <- data_frame0(
+    x = factor(rep(c("A", "C"), each = 3), c("A", "B", "C")),
+    y = 1:6
+  )
+
+  ld <- layer_data(
+    ggplot(df, aes(x, y)) + geom_violin() +
+      scale_x_discrete(drop = FALSE)
+  )
+
+  expect_s3_class(ld$x, "mapped_discrete")
+  expect_equal(unique(ld$x), c(1, 3))
+})