Add an option to add a diagonal line to a figure


While horizontal and vertical lines can be easily added to a figure using `fig.add_vline()` and `fig.add_hline()` diagonals are much more complicated. 

It would be great to implement a similar function that allows adding diagonals.

This could be called `fig.add_abline()` similar to the `geom_abline()` in `ggplot2`

Thanks for considering this.