NYC subway ridership trends mapped MAY 08 2009
Mike Frumin took the NYC subway ridership data from all the way back to 1905 (!!) and graphed it on a map, with a sparkline of the ridership data for each stop. Frumin explains the project a little more here.
The result, after much whacking, is, I think, compelling, but you'll have to see for yourself. The general idea it that the history of subway ridership tells a story about the history of a neighborhood that is much richer than the overall trend. An example, below, shows the wild comeback of inner Williamsburg, and how the growth decays at each successive stop away from Manhattan on the L train.
Update: Here's another representation of the same data. In this one, the ridership numbers are represented by the thickness of the subway line.