Add function support in lineplot

[mrkn]

[nanobowers]

@mrkn I have edits in progress and expect to create a PR later this week for support of functions in lineplot

[mrkn]

I have already touched this issue. It's stalled while I was working for Ruby 3.0. But I'll continue soon.

[nanobowers]

@mrkn I should have asked first before proceeding. I will put my branch on a shelf, and manually merge in documentation updates for lineplot (per #6 ) to your changes once they are merged. thanks!