Generate one zeroGrob for each panel.

Fixes #1445

Author: hadley

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 2.0.0.9000
 
+* Multipanel empty data is correctly plotted, rather than throwing an unhelpful 
+  error (#1445).
+
 * Eliminate spurious warning if you have a layer with no data and no aesthetics
   (#1451).
 

R/geom-.r

@@ -67,7 +67,10 @@ Geom <- ggproto("Geom",
   },
 
   draw_layer = function(self, data, params, panel, coord) {
-    if (empty(data)) return(list(zeroGrob()))
+    if (empty(data)) {
+      n <- nlevels(data$PANEL)
+      return(rep(list(zeroGrob()), n))
+    }
 
     # Trim off extra parameters
     params <- params[intersect(names(params), self$parameters())]