Think of it as a number of streams[1] running in parallel. The wider the stream, the more important the associated tag. Since the actual values are scaled to add up to 100%, comparing between relative importance is all they're good for.

There's also aa absolute version of this, which is a simple stacked area chart. It's not so useful here, because the relative popularity of tags is much more interesting than the absolute numbers, which are largely a function of StackOverflow getting more users in total. See for yourself: http://cl.ly/image/101e1y1q3i19

[1] There's also a thing called a "stream graph", which is related but not the same: http://en.wikipedia.org/wiki/Streamgraph