Users get to keep context since map visible underneath.
Challenges and decisions
Which data files
What color scale? Used Color Brewer as a starting point, but opacity meant that their colors weren't really right. Bumped up the saturation considerably, colors still hard to distinguish. Perhaps go for lots of indistinguishable steps?
What is the performance? Will this scale? How many resources would be needed to show the whole US, and at what cost?