In this post I will just explain the chart at the top of the page.
It shows the number of parliamentary seats each of the three main national parties gained in the UK 2010 General Election under first-past-the-post (FPTP) – these are the blue bars. The red bars show the number of seats each party would expect to gain under Alternative Vote (AV), based on a mock election involving 13,000 people. Finally the yellow bars show the number of seats which would be obtained under a proportional system.
The proportional system, where the number of seats is proportional to the number of votes gained nationwide, is what I would call “fair”.
Labour and Tory parties both benefit significantly under the current FPTP system and proposed AV systems.