sted from https://gist.githubusercontent.com/benmarwick/2a1bb0133ff568cbe28d/raw/fb53bd97121f7f9ce947837ef1a4c65a73bffb3f/geom_flat_violin.R
somewhat hackish solution to:
https://twitter.com/EamonCaddigan/status/646759751242620928
based mostly on copy/pasting from ggplot2 geom_violin source:
https://github.com/hadley/ggplot2/blob/master/R/geom-violin.r
The original seems to be: sourced from: https://gist.github.com/dgrtwo/eb7750e74997891d7c20,
Author is David Robinson.
A key internal function for the raincl