Improve stat_function documentation regarding xlim argument

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* Improve `stat_function` documentation regarding `xlim` argument. (@92amartins, #4474)
+
 * `qplot()` is now formally deprecated (@yutannihilation, #3956).
 
 * Use `rlang::hash()` instead of `digest::digest()`. This update may lead to 

R/geom-function.R

@@ -51,6 +51,16 @@
 #'
 #' base + geom_function(fun = f)
 #'
+#' # Using xlim to restrict the range of function
+#' ggplot(data.frame(x = rnorm(100)), aes(x)) +
+#' geom_density() +
+#' geom_function(fun = dnorm, colour = "red", xlim=c(-1, 1))
+#'
+#' # Using xlim to widen the range of function
+#' ggplot(data.frame(x = rnorm(100)), aes(x)) +
+#' geom_density() +
+#' geom_function(fun = dnorm, colour = "red", xlim=c(-7, 7))
+#'
 #' @export
 geom_function <- function(mapping = NULL, data = NULL, stat = "function",
                           position = "identity", ..., na.rm = FALSE,

R/stat-function.r

@@ -4,7 +4,7 @@
 #'   be vectorised.
 #' @param n Number of points to interpolate along the x axis.
 #' @param args List of additional arguments passed on to the function defined by `fun`.
-#' @param xlim Optionally, restrict the range of the function to this range.
+#' @param xlim Optionally, specify the range of the function.
 #' @section Computed variables:
 #' `stat_function()` computes the following variables:
 #' \describe{