don't fail when manual scales have missing values parameter. (#2916)

Author: clauswilke

R/scale-manual.r

@@ -101,8 +101,14 @@ scale_discrete_manual <- function(aesthetics, ..., values) {
 }
 
 
-manual_scale <- function(aesthetic, values, ...) {
-  force(values)
+manual_scale <- function(aesthetic, values = NULL, ...) {
+  # check for missing `values` parameter, in lieu of providing
+  # a default to all the different scale_*_manual() functions
+  if (rlang::is_missing(values)) {
+    values <- NULL
+  } else {
+    force(values)
+  }
 
   pal <- function(n) {
     if (n > length(values)) {