The Washington State Department of Transportation has published a map that details where Washington's 5,000-plus plug-in vehicles are registered.
Apparently, There Are 381 Public Charging Stations in the State of Washignton
This county-by-county breakdown is what we seek to obtain for all 50 states, though information as detailed as this is rarely published.
Olympia, the capital of Washington, is in Thurston County, which lists only 173 plug-in vehicle registrations.
Meanwhile, nearby Seattle (located in King County), the most populated city in Washington, lists an astronomical 3,048 plug-in vehicle registrations.
As the map rather accurately depicts, most plug-in vehicles are registered in urban and suburban areas. Most of Washington's outlying rural areas are home to few plug-in vehicles.
We suspect the sames would be true for most states in the US, though the official data simply does not exist to definitively say that's the case.