Remove all preprocessing from PlotAccessor.__call__
In https://github.com/pandas-dev/pandas/pull/28647, we discovered that `PlotAccessor.__call___` does quite a bit of preprocessing of the data before handing it off to the backend.

https://github.com/pandas-dev/pandas/blob/399212673ea8101a65d95830604edb633da0860d/pandas/plotting/_core.py#L728-L794

Depending on the plot kind and backend, *some* of this preprocesing may not be appropriate. The backend might want to see the "raw" data. That's not to say we shouldn't do any preprocessing though. We'll need some feedback from backend authors on what should be done before handing it off.