define "width" parameter for geom_boxplot. Closes #2893. (#2898)

Author: clauswilke

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 3.0.0.9000
 
+*   `geom_boxplot()` now understands the `width` parameter even when used with
+    a non-standard stat, such as `stat_identity()` (@clauswilke, #2893).
+
 *   `scale_*_date()`, `scale_*_time()` and `scale_*_datetime()` can now display 
     a secondary axis that is a __one-to-one__ transformation of the primary axis,
     implemented using the `sec.axis` argument to the scale constructor 

R/geom-boxplot.r

@@ -158,6 +158,11 @@ geom_boxplot <- function(mapping = NULL, data = NULL,
 #' @usage NULL
 #' @export
 GeomBoxplot <- ggproto("GeomBoxplot", Geom,
+
+  # need to declare `width`` here in case this geom is used with a stat that
+  # doesn't have a `width` parameter (e.g., `stat_identity`).
+  extra_params = c("na.rm", "width"),
+
   setup_data = function(data, params) {
     data$width <- data$width %||%
       params$width %||% (resolution(data$x, FALSE) * 0.9)