Automatic grouping of stocks for graphing

[ConradSelig]

#6 should be implemented before this ticket, but I'm not marking it as blocking as that is not strictly required.

Automatic groups should detect the highs and lows of every stock in this portfolio and try to best group stocks into a single graph so as to lose as little resolution as possible.

The user should be able to toggle automatic grouping on and off. The user should also be able to specify a "tolerance" value that will help determine how much resolution on graphs is willing to be lost when creating groups. Not sure exactly what this "tolerance" value will look like - so I'm just going to leave it up to whoever is brave enough to take on this ticket.

To be clear, automatic grouping should not be creating unnecessary groups. I would by far prefer displaying a dozen separate graph to the user than display a single graph so zoomed out that a stock appears as a flat line.