Fixes #1691. Positive and negative y-values are now calculated separately to allow stacking below the x-axis

Author: thomasp85

R/position-stack.r

@@ -62,13 +62,26 @@ PositionStack <- ggproto("PositionStack", Position,
       return(data)
     }
 
-    if (!is.null(data$ymin) && !all(data$ymin == 0))
-      warning("Stacking not well defined when ymin != 0", call. = FALSE)
+    if (!is.null(data$ymin) && !all((data$ymin == 0 & data$ymax >= 0) | data$ymax == 0 & data$ymin <= 0))
+      warning("Stacking not well defined when ymin and ymax is on opposite sides of 0", call. = FALSE)
 
     data
   },
 
   compute_panel = function(data, params, scales) {
-    collide(data, NULL, "position_stack", pos_stack)
+    negative <- if (!is.null(data$ymin)) data$ymin < 0 else rep(FALSE, nrow(data))
+    neg <- data[which(negative), ]
+    pos <- data[which(!negative), ]
+    if (any(negative)) {
+      # Negate group so sorting order is consistent across the x-axis.
+      # Undo negation afterwards so it doesn't mess up the rest
+      neg$group <- -neg$group
+      neg <- collide(neg, NULL, "position_stack", pos_stack)
+      neg$group <- -neg$group
+    }
+    if (any(!negative)) {
+      pos <- collide(pos, NULL, "position_stack", pos_stack)
+    }
+    rbind(pos, neg)
   }
 )