Support for Ternary plots

[Derek-Jones]

Potentially a rather big request.

One of the variables in my data has three possible values. A ternary plot would allow a graphical display of topical perspectives for a 3-valued variable.

Data, plus some analysis code, via here: https://shape-of-code.com/2022/01/23/including-natural-language-text-topics-in-a-regression-model/