Addresses #4476 clarifying that n is a computed value of stat_density() that can is not available as original mapping in after_stat. (#4832)

Author: thoolihan

R/aes-evaluation.r

@@ -23,10 +23,11 @@
 #' `after_stat()` replaces the old approaches of using either `stat()` or
 #' surrounding the variable names with `..`.
 #'
-#' @note Evaluation after stat transformation will only have access to the
-#' variables calculated by the stat. Evaluation after scaling will only have
-#' access to the final aesthetics of the layer (including non-mapped, default
-#' aesthetics). The original layer data can only be accessed at the first stage.
+#' @note Evaluation after stat transformation will have access to the
+#' variables calculated by the stat, not the original mapped values. Evaluation
+#' after scaling will only have access to the final aesthetics of the layer
+#' (including non-mapped, default aesthetics). The original layer data can only
+#' be accessed at the first stage.
 #'
 #' @param x An aesthetic expression using variables calculated by the stat
 #'   (`after_stat()`) or layer aesthetics (`after_scale()`).

R/stat-density.r

@@ -21,6 +21,7 @@
 #'   \item{count}{density * number of points - useful for stacked density
 #'      plots}
 #'   \item{scaled}{density estimate, scaled to maximum of 1}
+#'   \item{n}{number of points}
 #'   \item{ndensity}{alias for `scaled`, to mirror the syntax of
 #'    [`stat_bin()`]}
 #' }