Add midpoint example (#5896)

Author: teunbrand

R/scale-gradient.R

@@ -54,6 +54,14 @@
 #'   geom_point(aes(colour = z1)) +
 #'   scale_colour_gradientn(colours = terrain.colors(10))
 #'
+#' # The gradientn scale can be centered by using a rescaler
+#' ggplot(df, aes(x, y)) +
+#'   geom_point(aes(colour = z1)) +
+#'   scale_colour_gradientn(
+#'     colours = c("blue", "dodgerblue", "white", "orange", "red"),
+#'     rescaler = ~ scales::rescale_mid(.x, mid = 0)
+#'   )
+#'
 #' # Equivalent fill scales do the same job for the fill aesthetic
 #' ggplot(faithfuld, aes(waiting, eruptions)) +
 #'   geom_raster(aes(fill = density)) +