shapes_layer_below_traces mock: Make y and y2 gridlines visually distinct

Author: my-tien

test/image/mocks/zz-shapes_layer_below_traces.json

@@ -120,13 +120,15 @@
       }
     ],
     "yaxis": {
+      "gridcolor": "red",
       "insiderange": [
         "0",
         "215"
       ],
       "type": "linear"
     },
     "yaxis2": {
+      "gridwidth": 2,
       "side": "right",
       "overlaying": "y"
     }