Change k.Graph's data format to datum style.

This means that instead of an array of x/y pairs for each line on the graph (which duplicates data like the created times), there is an array of objects which represent a moment in time, and all the values at that time.

This assumes that each point in time will have a value for all of the data points, or at least most of them. Rickshaw does not make this assumption, but k.Graph now does, mainly because it has held true in all the data we have used so far.

This makes progress towards completing bug 865378.

r?