Choropleth Map Functionality

[adley-kim]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Other... Please describe:

Current behavior

The Map component currently supports a single fill color for all features.

Expected behavior

It would be nice if we could pass a property key as the fill value alongside colorScheme and emptyColor as props in order to render fill colors based on the designated property value of each feature.

Minimal reproduction of the problem with instructions

I wasn't able to come up with a complete test, but I think this feature can be implemented by adopting some of the logic in the HeatMap component.

The getValueScale() function used by HeatMapSeries would have to be adjusted to indicate which property to use as the value since geojson doesn't conform to the standard ChartDataShape.

The resulting valueScale() function could then be passed to the renderCountry() callback in order to apply individual fill colors to each path.

What is the motivation / use case for changing the behavior?

Mapping scaled fill values to individual paths would allow users to build choropleth maps, which offer more informative data viz than the current single-color basemaps.