I'm working on adding a new type of layer in uMap: heatmap. Here is a demo
When creating a new layer, you can now chose between three types: Default, Cluster or Heatmap (new). And you can still switch from one type to another once created.
When editing a heatmap layer, two new advanced properties will appear (both are optional):
heatmap radius: this allows you to override the default radius of the heatmap, which is by default 25px
heatmap intensity property: you can define a property of your feature (say a property of the geojson you have uploaded) to be used as intensity factor; for example, in the demo I've set up, I've added "CIRCONF" as intensity property, which is the circumference of each tree used of computing the heatmap.
Many thanks go to @mourner for the plugin Leaflet.heat.
Once again, this is a working progress easter egg, please test and give feedback!