From ba9748b63b40978fe6ec7432594478ad047c843b Mon Sep 17 00:00:00 2001
From: Hiroaki Yutani <yutani.ini@gmail.com>
Date: Fri, 6 Sep 2019 20:10:54 +0900
Subject: [PATCH 1/2] Add a failing test

---
 tests/testthat/test-stat-density.R | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tests/testthat/test-stat-density.R b/tests/testthat/test-stat-density.R
index 9c4791e337..ff3996bf44 100644
--- a/tests/testthat/test-stat-density.R
+++ b/tests/testthat/test-stat-density.R
@@ -12,3 +12,10 @@ test_that("compute_density returns useful df and throws warning when <2 values",
   expect_equal(names(dens), c("x", "density", "scaled", "ndensity", "count", "n"))
   expect_type(dens$x, "double")
 })
+
+test_that("stat_density throws error when y aesthetic is present", {
+  dat <- data_frame(x = 1:3, y = 1:3)
+
+  expect_error(ggplot_build(ggplot(dat, aes(x, y)) + stat_density()),
+    "must not be used with a y aesthetic.")
+})

From 2ab851248e265a96f951c19bf1aef94dc85f2a0d Mon Sep 17 00:00:00 2001
From: Hiroaki Yutani <yutani.ini@gmail.com>
Date: Fri, 6 Sep 2019 20:24:42 +0900
Subject: [PATCH 2/2] Raise a helpful error when there is a y aesthetic

---
 R/stat-density.r | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/R/stat-density.r b/R/stat-density.r
index 6d18b8bb12..f9f865af76 100644
--- a/R/stat-density.r
+++ b/R/stat-density.r
@@ -66,6 +66,13 @@ StatDensity <- ggproto("StatDensity", Stat,
   required_aes = "x",
   default_aes = aes(y = stat(density), fill = NA, weight = NULL),
 
+  setup_params = function(data, params) {
+    if (!is.null(data$y)) {
+      stop("stat_density() must not be used with a y aesthetic.", call. = FALSE)
+    }
+    params
+  },
+
   compute_group = function(data, scales, bw = "nrd0", adjust = 1, kernel = "gaussian",
                            n = 512, trim = FALSE, na.rm = FALSE) {
     if (trim) {