fix problems during ribbon data sort (#3235)

Author: thomasp85

R/geom-ribbon.r

@@ -63,7 +63,12 @@ GeomRibbon <- ggproto("GeomRibbon", Geom,
   required_aes = c("x", "ymin", "ymax"),
 
   setup_data = function(data, params) {
-    transform(data[order(data$PANEL, data$group, data$x), ], y = ymin)
+    if (is.null(data$ymin) && is.null(data$ymax)) {
+      stop("Either ymin or ymax must be given as an aesthetic.", call. = FALSE)
+    }
+    data <- data[order(data$PANEL, data$group, data$x), , drop = FALSE]
+    data$y <- data$ymin %||% data$ymax
+    data
   },
 
   draw_key = draw_key_polygon,