This map from the Economist illustrates pretty well the movement of population from middle America outwards from 2000-6. The message reaches us despite the large volume of data painted. (The gray shadow though was more than a little distracting.)
The map piqued my curiosity in two areas:
How did they determine the color scale? The average change over all counties (6.4%) was obviously used. Standard deviation was not since the ranges of change were unequal in size.
Was within-county percent change the best criterion? As is, an 80% drop in a 2,000-people county looks the same as an 80% drop in a 200,000-strong county.
Reference: "The Great Plains drain", Economist, Jan 17 2008.
PS. I am traveling and so posting will be limited.