Triangular Plot

http://www.ex-parrot.com/~chris/wwwitter/20050407-it_doesnt_matter_how_you_vote_either_way_your_planet_is_doomed.html
A triangular plot (also called a ternary plot, ternary graph, or de Finetti diagram) is a barycentric plot on three variables, which all add up to a constant. Graphically, a triangular plot is depicted as an equilateral triangle, with the three points of the triangle representing the three variables. Basically, the proportions of the variables a, b, and c, all must sum to some constant K (which is represented as 1.0 or 100%).
The triangular plot above shows the outcome of a vote between Liberal Democrats, Conservatives, and Labours, due to a recent opinion poll.