Integration of the Vega Leaflet plugin for displaying statistics

Vega is a visualization framework described at http://vega.github.io/ and which includes powerful choropleth map rendering. This issue is about integrating the Leaflet-Vega plugin into the application

The Leaflet Vega plugin adds a Vega layer on top of the map, and provides two way signaling between Leaflet and Vega. This way a complex Vega-based visualization can be added to a map without any additional JavaScript.

https://github.com/nyurik/leaflet-vega