Value by Alpha Election map with d3.js
The code herein describes how to create a "Value by Alpha" map. A thematic mapping technique that attempts to equalize the dominate visual variable; size, using partial opacities across classes.
The biggest take away is that you must create a basemap, then overlay the basemap using "fill-opacity" rather than rgba or rgb.
View example: http://geo-odyssey.com/links/choropleth/index.html