Bring stat_ydensity behavior in line with stat_density (#4112)

Fix #4111

* stat_ydensity drop groups with fewer than 2 + warn

This change is intended to bring behavior of stat_ydensity in line
with stat_density. Previously dropped with fewer than 3, now drops
with fewer than 2 and will display a warning message when dropping.

Author: andrewwbutler

NEWS.md

@@ -5,6 +5,8 @@
 
 * Updated style for example code (@rjake, #4092)
 
+* Only drop groups in `stat_ydensity()` when there are fewer than two data points and throw a warning (@andrewwbutler, #4111).
+
 # ggplot2 3.3.2
 This is a small release focusing on fixing regressions introduced in 3.3.1.
 

R/stat-ydensity.r

@@ -71,7 +71,10 @@ StatYdensity <- ggproto("StatYdensity", Stat,
 
   compute_group = function(data, scales, width = NULL, bw = "nrd0", adjust = 1,
                        kernel = "gaussian", trim = TRUE, na.rm = FALSE, flipped_aes = FALSE) {
-    if (nrow(data) < 3) return(new_data_frame())
+    if (nrow(data) < 2) {
+      warn("Groups with fewer than two data points have been dropped.")
+      return(new_data_frame())
+    }
     range <- range(data$y, na.rm = TRUE)
     modifier <- if (trim) 0 else 3
     bw <- calc_bw(data$y, bw)