<p>In the above picture of Portland, Ore., from Dustin Cable's breathtaking Racial Dot Map, each blue dot represents a white resident counted during the 2010 census. The city itself is about 76 percent white, making it the whitest big city in the U.S. And diversity has been dwindling in the neighborhoods close to the center of town, as minorities have increasingly moved out to the city's edges.</p>