Chicago's a great city for a bike-sharing service. It's pretty flat, and there are lots of wide roads with cycle lanes. I love Divvy and use it all the time. Not so much in the winter though: it gets very cold here. Nonetheless, this heat map of Divvy riders, created in R by Austin Wehrwein, reveals a hardcore set of riders that use the service even in the bitter cold. For example on January 27 2014 when the temperature was °F (-18°C), there were still 406 riders that day.
There's no calendar heatmap extension to ggplot2 (that I know of), but as Austin shows below it's actually very easy to create one in base ggplot2 simply by defining weekday, month and year columns in the data.
Austin Wehrwein: Heatmaps with Divvy Data