A little cleaning
Style (title and center)

Add a title and a subtitle, styled.
Center the map in the page.
Deal with missing values

Transform NA strings in the data into null values in the data maps
From NA to null

Treat the effects of missing values, previously represented as NA
strings, now as null
Dynamic scale

The domain of the quantize scale is recalculated every time the
variable to display is changed.
Merge pull request #1 from metroatlas/generalize-range-choropleth

Generalize range choropleth