From 0d2023d53ccbc88fc998b9c37b75f398d9671edf Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Tue, 12 Sep 2023 08:55:23 +0200
Subject: [PATCH] Violin resolution (#5402)

* Preserve mapped_discrete class

* Add test

* Add news bullet
---
 NEWS.md                             |  3 +++
 R/stat-ydensity.R                   |  5 ++++-
 tests/testthat/test-stat-ydensity.R | 16 ++++++++++++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/NEWS.md b/NEWS.md
index ea136bae78..28cb0695a5 100644
--- a/NEWS.md
+++ b/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)
 
diff --git a/R/stat-ydensity.R b/R/stat-ydensity.R
index a090898dea..c7f034bbd5 100644
--- a/R/stat-ydensity.R
+++ b/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
 
diff --git a/tests/testthat/test-stat-ydensity.R b/tests/testthat/test-stat-ydensity.R
index ecda132e04..98138d2d21 100644
--- a/tests/testthat/test-stat-ydensity.R
+++ b/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))
+})