fix fill ordering

- regl-line2d "skips" over "undefined"s, not "null"s

Author: etpinard

src/traces/scattergl/index.js

@@ -431,7 +431,7 @@ function plot(gd, subplot, cdata) {
         if(scene.fill2d) {
             scene.fillOptions = scene.fillOptions.map(function(fillOptions, i) {
                 var cdscatter = cdata[i];
-                if(!fillOptions || !cdscatter || !cdscatter[0] || !cdscatter[0].trace) return null;
+                if(!fillOptions || !cdscatter || !cdscatter[0] || !cdscatter[0].trace) return;
                 var cd = cdscatter[0];
                 var trace = cd.trace;
                 var stash = cd.t;

test/image/baselines/gl2d_fill-ordering.png

@@ -0,0 +1,40 @@
+{
+  "data": [
+    {
+      "x": [1, 2, 3, 4, 5, 6],
+      "y": [100, 100, 0, 0, 0, 0],
+      "fill": "tozeroy",
+      "type": "scattergl"
+    },
+    {
+      "x": [1, 2, 3, 4, 5, 6],
+      "y": [0, 0, 0, 100, 100, 0],
+      "fill": "tozeroy",
+      "type": "scattergl",
+      "mode": "none"
+    },
+    {
+      "x": [1, 2, 3, 4, 5, 6],
+      "y": [99, 99, 99, 100, 100, 100],
+      "type": "scattergl",
+      "mode": "lines+markers"
+    },
+    {
+      "x": [1, 2, 3, 4, 5, 6],
+      "y": [0, 0, 0, null, 50, 50],
+      "fill": "tozeroy",
+      "type": "scattergl",
+      "mode": "none"
+    },
+    {
+      "x": [1, 2, 3, 4, 5, 6],
+      "y": [100, 0, 0, 0, 0, 100],
+      "type": "scattergl",
+      "mode": "lines+markers"
+    }
+  ],
+  "layout": {
+    "margin": {"l": 40, "r": 50, "b": 80, "t": 40},
+    "legend": {"orientation": "h", "x": "0.5", "xanchor": "center"}
+  }
+}