DistributionDisplay Consistent Names

[AdamTheisen]

Remove graph from function names in DistributionDisplay as part of v2.0.0 release

Describe what you were trying to get done.
There are two functions in DistributionDisplay that include "graph" at the end which no other function does. We should remove that for consistency purposes.

plot_stacked_bar_graph
plot_stairstep_graph

We also need to review all functions in this module to ensure that it's returning the standard axes object as we do with all other plotting functions. The two above and at least one other returns a dictionary instead.