allow fewer elements in named values vector in manual scales

NEWS.md

@@ -1,5 +1,9 @@
 # ggplot2 (development version)
 
+* Manual scales now allow named vectors passed to `values` to contain fewer 
+  elements than existing in the data. Elements not present in values will be set
+  to `NA` (@thomasp85, #3451)
+
 * Add support for the BrailleR package for creating descriptions of the plot
   when rendered (@thomasp85, #4459)
 

R/scale-manual.r

@@ -141,7 +141,7 @@ manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ...) {
   }
 
   pal <- function(n) {
-    if (n > length(values)) {
+    if (is.null(names(values)) && n > length(values)) {
       abort(glue("Insufficient values in manual scale. {n} needed but only {length(values)} provided."))
     }
     values