`roundTop` display issues

[jaredramirez]

Hey there,

First off, thanks for the super rad package!

When using bar stacks with roundTop, I've come across two kind of obscure display issues when using CA.roundTop on C.bars within a stack

1. Only the topmost bar of the entire graph is rounded, rather than the topmost bar of that stack. In the following screenshot, I would expect the top of the only visible bar on the 9/16 to be rounded

2. Sometimes, when showing bars where some columns have very high values and others have very low, the rounded top produces unexpected results. In the case of the Week of Aug 23, there's a bit of the bar displayed below the x axis.

The code is roughly:

C.bars [ CA.roundTop 0.3 ]
     [ C.stacked [ ... ]
     ]
     data

If it'd be helpful, I can also re-create these issues in an Ellie.