Plotnine is heavily inspired by the ggplot2 library, which uses the + operator in the same way: https://ggplot2.tidyverse.org/#usage

> a new python plotting library

Whilst it's still not yet at 1.0.0, it's not that new: the first (0.1.0) release was in 2017: https://pypi.org/project/plotnine/#history

Because it's aimed at data scientists who would rather be using R...

Because most of the time this will be used is not part of a software development project but rather producing publication plots in a script or plots in a notebook. Not what you would want to do when incorporating it into a web app.

While it’s generally considered to be bad practice to import everything into the global namespace, I think it’s fine to do this in an ad-hoc environment such as a notebook as it makes using the many functions plotnine provides more convenient. An additional advantage is that the resulting code more closely resembles the original ggplot2 code. Alternatively, it’s quite common to `import plotnine as p9` and prefix every function with `p9`.

Disclaimer: I made the plotnine homepage and cheatsheet.