Stacked improvements

I hate stacked graphs

*Edit*

This selects graph stacks as spans by:

knocking back surrounding stacks

colouring the following line the same as the selected line

labelling the span with the value at that point

knocking back unselected keys

colouring the cursor line for the span the same as the line selected

it makes key, line and stack colours consistent

it prevents selection of a specific key when there is no data at this point

it shouldn't break anything else. I hope.

*Edit 2*

Because I've merged in master I think we might be stuck with the messy history.