For Region State, or Nation graphs you can ask that only counties with one of 3 ruralities be included in the data aggregations. By default all counties are included. The 3 ruralities are defined as:

• Metropolitan: a county having a large, core city with a population of 50,000 or more, and communities that adjoin the core city that are inter-related to it economically and socially.
• Micropolitan: a Non-Metropolitan county having at least one urban cluster of 10,000 but less than 50,000 population.
• Non-Metro-Non-Core: a Non-Metropolitan county having no urban cluster with population of more than 10,000.

In 2003 the Office of Management and Budget (OMB) assigned one of these ruralities to each of the Nation's counties based on the 2000 Census, where previously counties were designated either Metropolitan or Non-Metropolitan. County graphs indicate the rurality of that county. You can read about Measuring Rurality at this US Dept of Agriculture site where you can find discussion and links concerning rurality.