Cartograms distort the area of geographic features to reflect the values of an underlying variable. In this case, the area of each county in the contiguous 48 states has been changed to reflect the population claiming Irish ancestry. In the top map, both county size and coloring reflect proportion of the population that is Irish. In the bottom one, the area reflects the total population claiming Irish ancestry, which emphasizes large urban areas. Happy St. Patrick's Day!
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