Stacks hidable via legend and various hidable stack related bug fixes

(In reference to #379)

We had already made significant progress implementing this functionality in StackableChart. We think that it would be better to implement this in CompositeChart instead, and use CompositeChart to perform chart stacking (abolishing the GroupStack class), but this would be a pretty major refactor. Hopefully this functionality is adequate for now.

We also fixed a few bugs around chart coloring and titles when hideStack and showStack are used.