Add documentation on creating custom geoms, themes, etc.

[janfreyberg]

For ggplot2, there are many R packages that define new geom_* functions or new theme_* functions. I'd love to be able to define a new geom for plotnine (I'm particularly thinking of something similar to ggridges), but it's not clear to me how I would do that.

Maybe it'd be possible to add some developer instructions that tell you which functions to look at / which classes to inherit from when defining custom geoms.

thanks!

[has2k1]

Checkout the documentation for the base geom and stat classes and also look at how some geoms are implemented.

If figuring out how to "extend" in such basic ways is not obvious given the API documentation and current source code, then I think that is where the focus should be first, i.e make monkey see, monkey do easier.

[janfreyberg]

Thanks. I started by checking other geoms (eg. geom_point) but just saw a bunch of variables defined, so it was a bit confusing.

I'll take a look at the base classes and take it from there.

[has2k1]

As you go through the process, can you note down "why" questions that come to you but take long to figure out. Thanks.