add heatmap with '0' x/y category mock

Author: etpinard

test/image/baselines/heatmap-with-zero-category.png

@@ -0,0 +1,32 @@
+{
+  "data": [
+    {
+      "x": [ 3, 2, 1, 0 ],
+      "y": [ 3, 2, 1, 0 ],
+      "z": [
+        [ 98, 97, 96, null ],
+        [ 88, 87, 86, null ],
+        [ 78, 77, 76, null ],
+        [ null, null, null, null ]
+      ],
+      "type": "heatmap",
+      "colorscale": [
+        [ 0, "#3D9970" ],
+        [ 1, "#001f3f" ]
+      ],
+      "showscale": false,
+      "xgap": 1,
+      "ygap": 1
+    }
+  ],
+  "layout": {
+    "height": 450,
+    "width": 650,
+    "xaxis": {
+      "type": "category"
+    },
+    "yaxis": {
+      "type": "category"
+    }
+  }
+}