docs: Add example of how to create polar bar charts

[joelostblom]

I was playing around a bit and thought this was a potentially useful example of how to do something that is not immediately obvious when reading the arc_mark docs. In many other contexts these are referred to as "polar bar charts" so I chose that name to show that it is possible to create these in altair, although we are not technically supporting a polar coordinate system.

[mattijn]

Maybe add the name "rose plot" to it as well.
I did a section on this at the geopython conference last year, see also onwards from here: https://mattijn.github.io/posts/2023-03-07-talk-geopython-2023/#interaction

[joelostblom]

Nice post @mattijn ! Could you link me to where you defined the function utils_chart_rose()? Maybe I can borrow something from there to improve this example as well

[jonmmease]

Great example @joelostblom! Someday it would be great to have the logic for creating the polar guide included in a package somewhere. Either in Vega-Altair itself or an altair-polar package or something. We could probably also support points and lines in polar coordinates as well.

[mattijn]

Could you link me to where you defined the function utils_chart_rose()?

Its here: https://github.com/mattijn/talks/blob/main/docs/utils_geoconf_23.py#L35. Totally agree with @jonmmease that better support for this is welcome as there is currently no any automated axis positioning.

eg. sometimes these polar bars are not starting at zero, but has the middle line of the bar at zero, then you can use a theta offset as such alt.Chart().mark_arc(thetaOffset=np.deg2rad(-7.5)).encode(..) to accommodate for this.

[mattijn]

Its good enough already for now! Thanks @joelostblom!