Solar PV divert graph should be stacked area

[mattjgalloway]

Currently there’s a hack in place to make the diverted energy show above the house use energy. This is however a hack, whereby the divert graph is just drawn the size of the house use plus divert, and ideally it should show as a proper stack.

[mattjgalloway]

We can probably use a similar technique to here:
https://stackoverflow.com/questions/43342597/android-fill-the-color-between-two-lines-using-mpandroidchart/43452404#43452404