This service looks great, seems to be just like Graphite feature-wise, but without the clunkyness. The problem is that the pricing makes absolutely no sense and is hard to estimate.
In short a measurement is a single datapoint i.e. <key, value, timestamp>. So if you have a sense of how many metrics you want to track and at what frequency, it's relatively straight-forward to calculate the list price. There's an estimator on the pricing page to help you do that once you understand what constitutes a "measurement".
That said, librato seems like it would be nice if your stat volume was low.