Handle label functions that return character(0) (#3740)

Author: thomasp85

R/guides-axis.r

@@ -85,14 +85,6 @@ guide_train.axis <- function(guide, scale, aesthetic = NULL) {
     ticks$.value <- breaks
     ticks$.label <- scale$get_labels(breaks)
 
-    if (is.list(ticks$.label)) {
-      if (any(sapply(ticks$.label, is.language))) {
-        ticks$.label <- do.call(expression, ticks$.label)
-      } else {
-        ticks$.label <- unlist(ticks$.label)
-      }
-    }
-
     guide$key <- ticks[is.finite(ticks[[aesthetic]]), ]
   }
 

R/scale-.r

@@ -687,6 +687,18 @@ ScaleContinuous <- ggproto("ScaleContinuous", Scale,
     if (length(labels) != length(breaks)) {
       abort("Breaks and labels are different lengths")
     }
+    if (is.list(labels)) {
+      # Guard against list with empty elements
+      labels[vapply(labels, length, integer(1)) == 0] <- ""
+      # Make sure each element is scalar
+      labels <- lapply(labels, `[`, 1)
+
+      if (any(vapply(labels, is.language, logical(1)))) {
+        labels <- do.call(expression, labels)
+      } else {
+        labels <- unlist(labels)
+      }
+    }
 
     labels
   },