add violin plot

Add violin plot:

https://nbviewer.org/github/JetBrains/lets-plot-docs/blob/master/source/examples/cookbook/geom_violin.ipynb

Start by reading the linked JetBrains lets-plot geom_violin notebook and comparing its behavior with the existing Kotlin plotting API. No repository file, test, or entry point is named in the issue, so identify the corresponding plot implementation and tests before starting. Done means the library can produce a violin plot matching the referenced example and the relevant tests pass.

Written by the indexing model from the issue text.