Bar chart variants #1015

A couple of bar chart variants will be supported by #80 but only with extra calculation and redundant data. We could express these semantically fairly easily, I would think.

• Grouped waterfall:
  
  With explicit positioning this requires the user to calculate all the subtotals... can we just make this a new barmode? One complication is how to specify the total bar at the end, which ideally has no data associated with it but is a trace, so that it can be styled (and named) arbitrarily.
  Would we need a separate mode for a single waterfall group? I don't think so, and keeping each component as a separate trace adds styling and labeling flexibility.
• Grouped stacks:
  
  Seems like just another barmode plus per-trace attributes for group number or ID.