Click-n-drag show difference between points on graph

[tecosaur]

Hello,

I think it would be rather nice if graphs could show the change between two points, a 'la google stocks.

I think that picture does a perfect job of illustrating the feature.

Thanks for all the work you've done so far, and I'll hope I might see this at some point in the future :grin:

[yagebu]

A PR implementing this would be very welcome :)