new function offsetToZero

We've written a new function for the graphite renderer that we've been using internally for a while. offsetToZero subtracts the minimum value in a series from every datapoint in the series, essentially bringing the whole series down so that the lowest value is at zero. This is useful for example when we're measuring ping response times from various devices, some series are higher than others due to the number of network hops in the way which add latency. But we're interested in comparing the overall response times of all of our devices together, so this brings them all into line and clearly shows which ones are higher than they should be. Essentially what we're assuming is that one of the responses in the series is as fast as it can respond, and basing everything off of that. You assume that the lowest result represents the latency to the target and factor that out of all of the results. It's not 100% accurate but it's a common shortcut when measuring round trip times.

If you like the idea I'd be happy to write some user documentation or provide examples.