Lock map aspect ratio

[rahuljani]

Hey! Great vis. Before the latest update I was making multiple bookmarks to show/hide multiple heatmaps.

The "dynamic background image" update seems like it would fix this clunky method, however my heatmaps are different sizes and the vis currently stretches the background image to fill the vis window.

Is there any chance you can add a "lock aspect ratio" method? Or perhaps add "map_height|map_width" fields alongside the "map_url" so we can define the map size?