Here's a neat visualization of the decline of the British, Spanish, Portugese and French empires from 1800 to present day. It's definitely more art than stats -- judging by the relative size of India and Australia I think the circles are scaled to area, not population -- but it definitely does capture the drama and the ebb and flow of colonial history. One particularly smart aesthetic choice was to let the independent colonies fade out of the visualization and to focus on the dwindling empires themselves. Fast-forward to around 1915 to see what I mean.
via Andrew Sullivan: The decline of empires