image: put imagelayer underneath heatmap

Author: antoinerg

src/plots/cartesian/constants.js

@@ -63,6 +63,7 @@ module.exports = {
     // Layers to keep trace types in the right order
     // N.B. each  'unique' plot method must have its own layer
     traceLayerClasses: [
+        'imagelayer',
         'heatmaplayer',
         'contourcarpetlayer', 'contourlayer',
         'funnellayer', 'waterfalllayer', 'barlayer',

test/image/baselines/image_with_heatmap.png

@@ -0,0 +1,29 @@
+{
+  "data": [{
+    "type": "image",
+    "z": [
+      [
+        [255, 0, 0],
+        [0, 255, 0],
+        [0, 0, 255]
+      ],
+      [
+        [255, 0, 0],
+        [0, 255, 0],
+        [0, 0, 255]
+      ]
+    ]
+  }, {
+    "type": "heatmap",
+    "z": [
+      [-1, 0, 1]
+    ]
+  }],
+  "layout": {
+    "width": 400,
+    "height": 400,
+    "title": {
+      "text": "Image is below heatmap"
+    }
+  }
+}