The Congressional Dot Map is an attempt to illustrate some of the issues influencing current redistricting practices in the USA.
The map from the University of Virginia includes two separate dot map layers. The first layer displays one dot on the map for every Republican and Democratic vote in the 2008 election. The second dot map uses 2010 census block data to show the distribution of the population by race.
You can read more about the implications of the map for redistricting at StatChat.