Visualization improvement for heatmaps in geo-spatial features

[Firenze11]

Currently we have 2 ways to show numerical values over spatially distributed data:

1. Overlapping heatmaps, used for displaying two data slices on a map:

2. Hexagon binned heatmaps, used for displaying spatially aggregated 1-Dimensional data (e.g. values of one feature):

The drawbacks are:

1. Overlapping heapmaps might occlude information
2. It's hard to show multiple dimensions at the same time

We need to invent new visual encodings to fix it.

[Firenze11]

Inspirations: https://pair-code.github.io/interpretability/uncertainty-over-space/