At the time, you could only use a limited number of pre-packaged datasets. Bjorn has just announced the Thematic Mapping Engine API, a Javascript library that lets you do thematic mapping in the Google Earth plugin with your own data (including online sources like Google Docs Spreadsheet). Still in beta, and Bjorn is looking for people to try it out (non-commercial use only). Documentation is a bit sparse right now, but if you look at the page source for example maps created with the API, it’s not too hard to figure out. Read More
