Earlier quoted context omitted.
Perhaps it's pessimistic of me to say so, but I doubt Amazon wants you to be able to appreciate the price relationship with quite this degree of immediacy. Sure, the data is there for you to plot yourself or use a third-party tool such as the OP's. But if you naively select an on-demand instance (the default) where a dedicated instance would be more advantageous for you, Mr. Bezos is made a tiny bit happier.
Maybe, but the heavy reserved instances come with a required hourly commitment. If you buy one, you have to pay the 24/7 hourly usage fee regardless of whether you're actually running a server. That strikes me as the bigger "gotcha."
There is one more "gotcha" if you dig deeper. That is the drop in EC2 prices and Amazon in general has been vague about passing on the benefits of price drop to old reservations ( https://forums.aws.amazon.com/message.jspa?messageID=325153 ) . We even thought about adding speculative price drop and then do the comparison, but dropped the idea. It will be interesting to hear the take of HN community on this ..