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: Someone made an article comparing the two here: http://pltn.ca/plotnine-superior-python-ggplot/