[heatmap] fix default sorting (#3450)

Currently the heatmap axis are sorted randomly, this PR makes it such
that it's properly sorted.

Also allowing for specifying the left and bottom margins

superset/assets/javascripts/explore/stores/visTypes.js

@@ -943,6 +943,7 @@ export const visTypes = {
           ['xscale_interval', 'yscale_interval'],
           ['canvas_image_rendering'],
           ['normalize_across'],
+          ['left_margin', 'bottom_margin'],
         ],
       },
     ],