Add ridge plot

[MillenniumFalconMechanic]

From original requirements doc:

Highchart Ridge plot https://www.highcharts.com/blog/tutorials/density-chart/.

Example in a R tool called seurat:
https://satijalab.org/seurat/articles/visualization_vignette.html

This view is useful for looking at 1) distributions of a set of continuous features 2) a single continuous feature across many categories.

Ridge plot represents the same data as in bookmarks here (no category, a set of continuous features) and here (a single continuous feature, several categories), but in a different view. In fact, this is to rotate side-by-side violin plot graph on its side and remove the bottom half of the violin plot. Xena already has the kernel density estimation code. The team needs to decide on whether to use multi-color, then color scale.