Mike Frumin has put together a compelling map of NYC subway station ‘registrations’ (that is, the number of people who enter the station). The maps show spark lines of ridership over time, which provides a rich understanding of what has happened in neighborhoods during different periods. The interactive map is well-worth a visit.

Ridership around Wall Street, for example, closely tracks swings in the economy:

 

The back story:

My first stab
at visualizing this data was a traditional cartographic approach,
showing the overall growth from 1977 to 2006 at each station. This told
an approximate story at the level of the whole the city, but did not
leave much room for detailed exploration. Thanks to geoserver’s awesome new(ish) dynamic symbolizers functionality, it was trivial to plot the station-by-station time series sparklines (generated in R of course) onto the interactive online map…

For all this, I never really felt like this little experiment was ready for an audience. That all changed when OpenGeo put up its Open StreetMap base layer for the web, giving fancy overlays like this one the context they need.