Allow explicit setting of yMax on horizon chart

[caged]

I have a use case where I'm measuring the same metric across a bunch of different machines (e.g. the number of git operations in the last two hours across 5 file servers). I'd like to be able to compare them to each other, but currently each horizon's chart yMax is determined by its own dataset. I'd like to be able to normalize the scale across a bunch of horizons by explicitly setting the yMax.

Here's an example of the problem. Those spikes in fs8a shouldn't be near as pronounced because they are relatively non-eventful when compared with what's happening on fs9a.

[mbostock]

This seems like the perfect use-case for http://square.github.io/cubism/; have you tried it? Cubism horizon charts let you specify an extent (ymin/ymax).