fix stacked scatter for groupby traces

Author: etpinard

src/traces/scatter/calc.js

@@ -38,8 +38,9 @@ function calc(gd, trace) {
     var yAttr = 'y';
     var posAttr;
     if(stackGroupOpts) {
-        stackGroupOpts.traceIndices.push(trace.index);
+        Lib.pushUnique(stackGroupOpts.traceIndices, trace._expandedIndex);
         isV = stackGroupOpts.orientation === 'v';
+
         // size, like we use for bar
         if(isV) {
             yAttr = 's';

test/image/baselines/stacked_area_groupby.png

@@ -0,0 +1,18 @@
+{
+  "data": [
+    {
+      "x": [ 1, 1, 2, 2, 3, 3 ],
+      "y": [ 1, 1, 2, 2, 3, 3 ],
+      "stackgroup": 1,
+      "transforms": [
+        {
+          "type": "groupby",
+          "groups": [ "a", "b", "a", "b", "a", "b" ]
+        }
+      ]
+    }
+  ],
+  "layout": {
+    "width": 400
+  }
+}