Improve documentation for scale_*_gradient (#3407, fixes #3299)

* Add examples for na.value = NA

* Addressed PR documentation changes

Author: Megan Beckett

R/scale-gradient.r

@@ -29,6 +29,13 @@
 #'   z2 = abs(rnorm(100))
 #' )
 #'
+#' df_na <- data.frame(
+#'   value = seq(1, 20),
+#'   x = runif(20),
+#'   y = runif(20),
+#'   z1 = c(rep(NA, 10), rnorm(10))
+#' )
+#'
 #' # Default colour scale colours from light blue to dark blue
 #' ggplot(df, aes(x, y)) +
 #'   geom_point(aes(colour = z2))
@@ -54,6 +61,16 @@
 #'   scale_colour_gradient(low = "white", high = "black")
 #' # Avoid red-green colour contrasts because ~10% of men have difficulty
 #' # seeing them
+#'
+#'# Use `na.value = NA` to hide missing values but keep the original axis range
+#' ggplot(df_na, aes(x = value, y)) +
+#'   geom_bar(aes(fill = z1), stat = "identity") +
+#'   scale_fill_gradient(low = "yellow", high = "red", na.value = NA)
+#'
+#'  ggplot(df_na, aes(x, y)) +
+#'    geom_point(aes(colour = z1)) +
+#'    scale_colour_gradient(low = "yellow", high = "red", na.value = NA)
+#'
 scale_colour_gradient <- function(..., low = "#132B43", high = "#56B1F7", space = "Lab",
                                   na.value = "grey50", guide = "colourbar", aesthetics = "colour") {
   continuous_scale(aesthetics, "gradient", seq_gradient_pal(low, high, space),