At least with unemployment rates, population, and a few other public data sets, that's now possible. Using a technology acquired from Gapminder and its Trendalyzer tool, typing in something like Unemployment rate Sonoma County, CA returns a graph that shows historical data and can be narrowed by state and county results. You can also grab a direct link to your comparison to create an auto-updating reference point. Neat little hack, and we hope it gets expanded soon.