Improve labeller() behavior for lookup tables

Author: 92amartins

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* `labeller()` now handles unspecified entries from lookup tables
+  (@92amartins, #4599).
+
 * `fortify.default()` now accepts a data-frame-like object granted the object
   exhibits healthy `dim()`, `colnames()`, and `as.data.frame()` behaviors
   (@hpages, #5390).

R/labeller.R

@@ -450,7 +450,11 @@ labeller <- function(..., .rows = NULL, .cols = NULL,
       # Apply named labeller one by one
       out <- lapply(names(labels), function(label) {
         if (label %in% names(labellers)) {
-          labellers[[label]](labels[label])[[1]]
+          # Yield custom labels with any NA replaced with original
+          lbls <- labellers[[label]](labels[label])[[1]]
+          ind <- which(is.na(lbls))
+          lbls[ind] <- .default(labels[label])[[1]][ind]
+          lbls
         } else {
           .default(labels[label])[[1]]
         }