META: bar type improvements / changes before removing 'experimental' tag

[hamilton]

The hope for 2.11 is to have bars and points relatively feature-complete and well-tested. This has been a gaping hole in the library since day one. Here are some remaining issues:

• rollovers for bar charts should mirror points and lines as much as possible
• legends for bar charts should not be plotted by default
• grouped bar charts need an option to not be colored by default
• ungrouped bar charts need an option to be colored
• we need to marry the histogram type to the bar type. A bar where one axis is numerical should appear like the current histogram type
• given the previous bullet, we need to make it so we can easily orient numerical-only bars either vertically or horizontally
• bar thickness needs to be something we can easily set
• we need to re-implement the other bullet-like parts of bars, including middle lines, goal ticks, and the like (categorical plots: add lane-specific accessors for predictors, ticklines, etc. #699)
• we need to have a way to support sub-label accessing (a secondary label underneath a categorical one which can provide more information)