Plotnine is quite nice. Its api is very similar to R's ggplot so it gives you the same level of expressiveness and flexibility while being able to stay the Python ecosystem to use numpy, pandas, etc..
I too am interested in what the differences are. I have used the yhathq ggplot library for a while and it is quite useful but I sometimes find it lacking certain types of plots and documentation.
Also the last commit to the yhathq ggplot library was on Nov 20, 2016, so this library looks like it is currently more active in development.
EDIT: User dpedu put up a video of it in their comment here: https://news.ycombinator.com/item?id=21853137