the coarse ones block out the fine ones. if there's a hot chat on your block, you probably won't see that because your region occludes it
Discussion
Try out https://nostr.info/geohash.html
You can zoom in on your block and hide the bigger squares. It should eventually hide the bigger squares depending on the zoom level and screen size automatically.
Oh, that's cool. I would make the top level a calmer color so that the next levels punch through more, but the count really helps. Looks good!