833 stacked area

Redirecting PR #851 to target develop branch
hmm since this code is just repeated from stackedBar, could we factor this out into `StackedPlot` extends `NewStylePlot` which is then extended by `StackedBar` and `StackedArea`? that will avoid repeittion of some sketchy code here (what's the deal with this yScale return stuff rite?). if not the extension approach... then use a mixin