I'll echo akamaka -- I like your graphs far more than timf's flash-based ones.

You might like to consider adding some indication (dotted lines, perhaps) of what the hourly rate for reserved instances is, since this should be approximately a floor on the hourly spot rate.

EDIT: Also, please adjust the vertical scaling based on the set of instances shown -- if I'm only looking at the price history for m1.small, it's silly to have it squished into the bottom 5% of the graph.