Server Sided Marker Clustering with Leaflet and Node/Mongo (blog.cintric.com)

I would think another way to avoid the grid problem is to use center point of all the points included in the count. So after grouping by your "chopped" value, you can pretty cheaply calculate the center of that group.

Awesome. We worked with something like this at my previous startup. Looks like something we really could have used.