Guide for County Based Heat Maps?

@ChrisH Perhaps it's the ID that is messing things up, not sure. The US states also has properties, ex ,"properties":{"STATE":"ID","NAME":"Idaho"}

Play around in http://geojson.io and see if you can cleanup the GeoJSON.
Also recommend https://mapshaper.org

Otherwise try searching for different GeoJSON maps out there. There's usually many sources.