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.
Suggestion -- use small multiples showing the data center prices on one screen, probably by making the graphs 1/4 size. Place your explanation in the space left empty since there's only (currently) 3 centers.
"A small multiple design consists of a single design repeated several times within the eyespan, each example showing a different value of the independent variable(s)."
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.