HOW to depict Britain’s extraordinary North-South political divide? An article in this week’s issue did so with words, but showing it graphically was slightly harder. That is because identifying the political affiliation of parliamentary constituencies by colour on an ordinary, geographic map doesn’t quite work: not all constituencies are the same area, though each represents roughly the same number of people. As a result, expansive rural constituencies appear far larger than small but densely-populated urban ones. Thus, a geographic map appears very blue (for Conservative) because it over-emphasises rural constituencies. And Liberal-Democrats (in yellow) look as if they hold sway over one-fifth of the country, when in fact it is visually skewed by some big, sparsely-populated places. Labour's presence seems meagre.

Our remedy was an equal-area "Dorling" cartogram (below), named after Danny Dorling of the University of Sheffield. Part map, part graph, it let us depict every constituency at the same size, while keeping them in the approximate position and retaining the overall shape of the country. Because the variations in size of rural and urban constituencies are eliminated, the eye gets a far better sense of the actual distribution of political party representation. Then we added another layer of information: the strength of political support, by shade. We included the geographical map as an inset so readers could have a visual link to the true shape and proportion of the constituencies, and could compare the two approaches.