Warn when contour generates no data.

Fixes #316

Author: hadley

R/plot-render.r

@@ -22,6 +22,8 @@ ggplot_gtable <- function(data) {
   theme <- plot_theme(plot)
 
   build_grob <- function(layer, layer_data) {
+    if (nrow(layer_data) == 0) return()
+
     dlply(layer_data, "PANEL", function(df) {
       panel_i <- match(df$PANEL[1], panel$layout$PANEL)
       layer$make_grob(df, scales = panel$ranges[[panel_i]], cs = plot$coord)

R/stat-contour.r

@@ -72,7 +72,10 @@ StatContour <- proto(Stat, {
     cl <- contourLines(
       x = sort(unique(data$x)), y = sort(unique(data$y)), z = z, 
       levels = breaks)  
-    if (length(cl) == 0) return(data.frame())
+    if (length(cl) == 0) {
+      warning("Not possible to generate contour data", call. = FALSE)
+      return(data.frame())
+    }
 
     cl <- lapply(cl, as.data.frame)    
     contour_df <- rbind.fill(cl)