Feature request: allow non-symmetrical violint plots

Hi! I love violinplots, but I always found that it might not be needed to plot them as symmetrical, the information is doubly present for no real reason. I will send a tiny pull request that adds a key-word argument nonsymmetrical to seaborn.violint, it can be False (the default), or "left" or "right" (or "bottom" or "top") to allow to only plot half of the violin.

Here are two tiny demos, for a single violin or more violins, for horizontal (top/bottom) or vertical (left/right) cases:

[givenx]

kind='kde'to sns.catplot would be perfect. We have had multiple requests from our clients to drop the symmetry as it can be off-putting. I think it is more important than you realise, @mwaskom, imho.

[njvack]

For what it’s worth, I’m looking for half-violin plots for Raincloud plots. That package has its own implementation but… it’s mainly a copy of your code.

You can jigger seaborn to do half-violins by adding fake off-scale data to yourset and abusing the hue parameter, but that’s pretty gross.

A more useful alternative, though, might be to add kind='kde'to sns.catplot — really, a violin plot is just two kdeplots pasted together.

Thoughts?