New chart type: pie chart

[anexpn]

Something like these:
https://github.com/piotrmurach/tty-pie
https://docs.rs/piechart/latest/piechart/

[kojix2]

I am a fan of Piotr Murach and TTY, but I don’t want to implement pie charts.

1. UnicodePlots.jl and UnicodePlot.rb do not have pie charts.
2. Pie charts are not well suited to ASCII-art-style rendering.
3. Several data visualization experts also argue that pie charts should generally be avoided.

That said, YouPlot currently does not have enough support for showing proportions with color grouping. I want to acknowledge that.