Fix after_scale() with plot mappings (#4265)

* Use plot$mapping in compute_geom_2()

* Use default_mapping in guide_geom()

* Merge mapping in setup_layer()

* Stop merging a mapping repeatedly

* Revert compute_geom_2()

* Migrate a comment

* Add a NEWS item

Author: yutannihilation

NEWS.md

@@ -34,6 +34,9 @@
 * Improved error with hint when piping a `ggplot` object into a facet function
   (#4379, @mitchelloharawild).
 
+* Fix a bug that `after_stat()` and `after_scale()` cannot refer to aesthetics
+  if it's specified in the plot-global mapping (@yutannihilation, #4260).
+
 # ggplot2 3.3.3
 This is a small patch release mainly intended to address changes in R and CRAN.
 It further changes the licensing model of ggplot2 to an MIT license.

R/layer.r

@@ -203,16 +203,18 @@ Layer <- ggproto("Layer", NULL,
   # hook to allow a layer access to the final layer data
   # in input form and to global plot info
   setup_layer = function(self, data, plot) {
+    # For annotation geoms, it is useful to be able to ignore the default aes
+    if (isTRUE(self$inherit.aes)) {
+      self$mapping <- defaults(self$mapping, plot$mapping)
+      # defaults() strips class, but it needs to be preserved for now
+      class(self$mapping) <- "uneval"
+    }
+
     data
   },
 
   compute_aesthetics = function(self, data, plot) {
-    # For annotation geoms, it is useful to be able to ignore the default aes
-    if (self$inherit.aes) {
-      aesthetics <- defaults(self$mapping, plot$mapping)
-    } else {
-      aesthetics <- self$mapping
-    }
+    aesthetics <- self$mapping
 
     # Drop aesthetics that are set or calculated
     set <- names(aesthetics) %in% names(self$aes_params)
@@ -289,9 +291,6 @@ Layer <- ggproto("Layer", NULL,
 
     # Assemble aesthetics from layer, plot and stat mappings
     aesthetics <- self$mapping
-    if (self$inherit.aes) {
-      aesthetics <- defaults(aesthetics, plot$mapping)
-    }
     aesthetics <- defaults(aesthetics, self$stat$default_aes)
     aesthetics <- compact(aesthetics)