Allow arbitrary mathematical operations on series/groups of series.

[EvanKrall]

Occasionally I'd like to plot some function of two series, like cpu usage / access rate, or the difference between series. It'd be nice to be able to do arbitrary mathematical operations on series.

At the very least, simple binary operations like log, +, -, *, / between two series would be useful. Bonus points for being able to write expressions. More bonus points if you implement a delay operator.

Graphite can do tons of neat calculations: http://graphite.readthedocs.org/en/1.0/functions.html. Maybe we can just provide this for the Graphite data provider.