Bivariate choropleth maps measures two types of data in choropleth style (areal units) simultaneously. The map above shows the percentage of votes for John Kerry in the 2004 presidential election versus each county's population in the year 2000. There are two color continuum's, one for population and one for the percentage of votes for Kerry. The places with overlap show how population and a high percentage of Kerry voters, the places that are green show high percentage of Kerry voters, but low population. The purple areas just show high population. The white areas are neither higher populated nor high percentage vote areas for Kerry.
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