Add adjust param to stat_density2d() (fix tidyverse#2860)

Author: haleyjeppson

NEWS

@@ -6,6 +6,8 @@ ggplot2 1.0.1
 * Improvements to order of colours and legend display for continuous colour
   scales extracted from colorbrewer palettes by `scale_*_distiller()` (@jiho, 1076)
 
+* `stat_density2d()` can now take an `adjust` parameter to scale the default bandwidth. (#2860, @haleyjeppson)
+
 ggplot2 1.0.0
 ----------------------------------------------------------------
 

R/stat-density-2d.r

@@ -5,6 +5,10 @@
 #' @param n number of grid points in each direction
 #' @param h Bandwidth (vector of length two). If `NULL`, estimated
 #'   using [MASS::bandwidth.nrd()].
+#' @param adjust A multiplicate bandwidth adjustment to be used if 'h' is
+#'    'NULL'. This makes it possible to adjust the bandwidth while still
+#'    using the a bandwidth estimator. For example, `adjust = 1/2` means
+#'    use half of the default bandwidth.
 #' @section Computed variables:
 #' Same as [stat_contour()]
 #'
@@ -19,6 +23,7 @@ stat_density_2d <- function(mapping = NULL, data = NULL,
                             contour = TRUE,
                             n = 100,
                             h = NULL,
+                            adjust = c(1, 1),
                             na.rm = FALSE,
                             show.legend = NA,
                             inherit.aes = TRUE) {
@@ -35,6 +40,7 @@ stat_density_2d <- function(mapping = NULL, data = NULL,
       contour = contour,
       n = n,
       h = h,
+      adjust = adjust,
       ...
     )
   )
@@ -54,11 +60,12 @@ StatDensity2d <- ggproto("StatDensity2d", Stat,
 
   required_aes = c("x", "y"),
 
-  compute_group = function(data, scales, na.rm = FALSE, h = NULL,
+  compute_group = function(data, scales, na.rm = FALSE, h = NULL, adjust = c(1, 1),
                            contour = TRUE, n = 100, bins = NULL,
                            binwidth = NULL) {
     if (is.null(h)) {
       h <- c(MASS::bandwidth.nrd(data$x), MASS::bandwidth.nrd(data$y))
+      h <- h * adjust
     }
 
     dens <- MASS::kde2d(