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

@@ -121,7 +121,7 @@ scale_discrete_manual <- function(aesthetics, ..., values, breaks = waiver()) {
 }
 
 
-manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ...) {
+manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), limits = NULL, ...) {
   # check for missing `values` parameter, in lieu of providing
   # a default to all the different scale_*_manual() functions
   if (is_missing(values)) {
@@ -130,6 +130,10 @@ manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ...) {
     force(values)
   }
 
+  if (is.null(limits)) {
+    limits <- names(values)
+  }
+
   # order values according to breaks
   if (is.vector(values) && is.null(names(values)) && !is.waive(breaks) &&
       !is.null(breaks) && !is.function(breaks)) {
@@ -146,5 +150,5 @@ manual_scale <- function(aesthetic, values = NULL, breaks = waiver(), ...) {
     }
     values
   }
-  discrete_scale(aesthetic, "manual", pal, breaks = breaks, ...)
+  discrete_scale(aesthetic, "manual", pal, breaks = breaks, limits = limits, ...)
 }