Add an example for stat_align() (#4940)

Author: yutannihilation

R/geom-ribbon.r

@@ -41,6 +41,22 @@
 #' h +
 #'   geom_ribbon(aes(ymin = level - 1, ymax = level + 1), fill = "grey70") +
 #'   geom_line(aes(y = level))
+#'
+#' # The underlying stat_align() takes care of unaligned data points
+#' df <- data.frame(
+#'   g = c("a", "a", "a", "b", "b", "b"),
+#'   x = c(1, 3, 5, 2, 4, 6),
+#'   y = c(2, 5, 1, 3, 6, 7)
+#' )
+#' a <- ggplot(df, aes(x, y, fill = g)) +
+#'   geom_area()
+#'
+#' # Two groups have points on different X values.
+#' a + geom_point(size = 8) + facet_grid(g ~ .)
+#'
+#' # stat_align() interpolates and aligns the value so that the areas can stack
+#' # properly.
+#' a + geom_point(stat = "align", position = "stack", size = 8)
 geom_ribbon <- function(mapping = NULL, data = NULL,
                         stat = "identity", position = "identity",
                         ...,