Improve free scale + bad coord error message

Fixes #2049

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 2.2.1.9000
 
+* `facet_wrap()` and `facet_grid()` both give better error messages if you
+  attempt to use an unsupported coord with free scales (#2049)
+
 * ggplot2 now works on R 3.1 onwards.
 
 * `facet_grid()` gives a more informative error message if you try to use

R/facet-.r

@@ -422,3 +422,21 @@ render_strips <- function(x = NULL, y = NULL, labeller, theme) {
     y = build_strip(y, labeller, theme, FALSE)
   )
 }
+
+
+check_coord_freedom <- function(coord) {
+  # Check first element of class vector because this is a hideous hack on
+  # top of another hideous hack
+  class <- class(coord)[[1]]
+
+  if (class %in% c("CoordCartesian", "CoordFlip")) {
+    return()
+  }
+
+  stop(
+    "Free scales are only supported with `coord_cartesian()` and `coord_flip()`",
+    call. = FALSE
+  )
+}
+
+

R/facet-grid-.r

@@ -275,6 +275,9 @@ FacetGrid <- ggproto("FacetGrid", Facet,
     data[order(data$PANEL), , drop = FALSE]
   },
   draw_panels = function(panels, layout, x_scales, y_scales, ranges, coord, data, theme, params) {
+    if (params$free$x || params$free$y)
+      check_coord_freedom(coord)
+
     cols <- which(layout$ROW == 1)
     rows <- which(layout$COL == 1)
     axes <- render_axes(ranges[cols], ranges[rows], coord, theme, transpose = TRUE)

R/facet-wrap.r

@@ -187,11 +187,9 @@ FacetWrap <- ggproto("FacetWrap", Facet,
     data[order(data$PANEL), ]
   },
   draw_panels = function(panels, layout, x_scales, y_scales, ranges, coord, data, theme, params) {
-    # If coord is non-cartesian and (x is free or y is free)
-    # then throw error
-    if ((!inherits(coord, "CoordCartesian")) && (params$free$x || params$free$y)) {
-      stop("ggplot2 does not currently support free scales with a non-cartesian coord", call. = FALSE)
-    }
+    if (params$free$x || params$free$y)
+      check_coord_freedom(coord)
+
     if (inherits(coord, "CoordFlip")) {
       if (params$free$x) {
         layout$SCALE_X <- seq_len(nrow(layout))