US average: 6.7
Top 5 states: VT 32.7, DC 19.8, HI 17.5, OR 15.2, CO 13.6
Bottom 5 states: AR 2.6, AK 2.2, MS 2.1, LA 1.9, OK 1.7
Population data from Wikipedia.
Charging stations data from:
Alternative Fuels Data Center: Electric Vehicle Charging Station Locations