plotnine is an implementation of a grammar of graphics in Python, it is based on ggplot2. The grammar allows users to compose plots by explicitly mapping data to the visual objects that make up the plot.
Plotting with a grammar is powerful, it makes custom (and otherwise complex) plots are easy to think about and then create, while the simple plots remain simple.
To find out about all building blocks that you can use to create a plot, check out the documentation. Since plotnine has an API similar to ggplot2, where we lack in coverage theggplot2 documentation may be of some help.