Require x *or* y in stat_ydensity() (#6645)

* populate x/y with 0 if absent

* add news bullet

Author: teunbrand

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* `stat_ydensity()` now only requires the `x` or `y` aesthetic. The other will
+  be populated with 0, similar to `stat_boxplot()` (@teunbrand, #6600)
 * Implemented `as.list()` and `S7::convert()` methods for lists and classes in ggplot2 
   (@teunbrand, #6695)
 * The default linetype in `geom_sf()` is derived from `geom_polygon()` for 

R/stat-ydensity.R

@@ -4,7 +4,7 @@
 #' @export
 StatYdensity <- ggproto(
   "StatYdensity", Stat,
-  required_aes = c("x", "y"),
+  required_aes = "x|y",
   non_missing_aes = "weight",
 
   setup_params = function(data, params) {
@@ -23,6 +23,12 @@ StatYdensity <- ggproto(
     params
   },
 
+  setup_data = function(self, data, params) {
+    var <- flipped_names(flip = params$flipped_aes)$x
+    data[[var]] <- data[[var]] %||% 0
+    data
+  },
+
   # `draw_quantiles` is here for deprecation repair reasons
   extra_params = c("na.rm", "orientation", "draw_quantiles"),