Plan to reintroduce fitted models on plots (apart from `PolyFit`)

[Rabeez]

The functional API (regplot) had the option to draw logistic, robust or lowess regressions via statsmodels but the new object API only offers polynomial fit with PolyFit.

Is this an intentional choice or will these be added as extensions?

This gap feels odd since the related Agg and Est stats have been improved in terms of flexibility.