Created via the U.S. Atlas project as
make topo/ky-counties.json, after modifying the Makefile to include population and area statistics. These per-county values are then used to compute the population density of each county.
topojson \ --id-property=FIPS,STATE_FIPS \ -p name=COUNTY,name=STATE,+population,area=+SQUARE_MIL \ -e KY-counties-pop.csv \ -- shp/ky/counties.shp shp/ky/states.shp
Colors by ColorBrewer.