Mosaic Cartograms in R

Cartograms — maps showing statistics — are powerful tools for visualising geographic data. For electoral statistics, areas like constituencies often ‘count’ for the same amount. In the United Kingdom, each constituency elects one member of parliament each. States in the US republic have differing numbers of electoral college votes.

We could draw a map, matching each constituency or state to its electoral ‘value’ with a regular shape. This is a mosaic cartogram. Its name comes from the pottery formed by placing lots of little pieces together.

Hexagons are a common shape for this purpose. It results in a map which looks like the 1980s British game show Blockbusters.

Mosaic cartograms can distort geography. Suppose you are keeping to the rule of one hex meaning one constituency. That means you cannot show one area split between an island and the mainland. Likewise, there is no proper way to show a constituency which surrounds another. That used to be the case with the Bath and North East Somerset constituencies. Bath was the ‘jam’ to the outer ‘doughnut’.

Due to creative border-drawing, districts in United States elections can form strange shapes. That also represents a challenge for the mosaic builder.

The benefit of showing each district in proportion can outweigh these limitations. Building mosaic cartograms may take much effort, even with computer help. It is important to credit people who create these maps and make the code available for analysts to use.