Ridges / Half-violins

[teunbrand]

I think at this point the data visualisation has become sort of a staple in the community and implementation-wise it is only an incremental change. We should add an option to render only one of the two sides of a violin. Horizontally, this allows for ridgeline plots.

[clauswilke]

Agreed. Additionally, people often stitch two half-violins together to show a contrasts between two distributions. It's worth thinking this through. Can this be done with a single call to geom_violin() or does it require two separate geoms (one for the right part and one for the left part)?

[teunbrand]

Yes, good call about the composite violin. It should definitely be possible.
In my mind, it would look something like this:

library(ggplot2)

ggplot(penguins, aes(species, bill_len, fill = sex)) +
  geom_violin(
    data = ~subset(.x, sex == "female"),
    side = "right"
  )
  geom_violin(
    data = ~subset(.x, sex == "male"),
    side = "left"
  )

The reason I wouldn't want to do aes(side = foo) is that foo may have >2 levels, in which case it is unclear what should happen. In addition, one would then also still need to map levels to sides.

Happy to have my mind changed though.

[teunbrand]

The way to construct a ridgeline plot would then be:

library(ggplot2)

ggplot(penguins, aes(bill_len, species)) +
  geom_violin(side = "top", width = 2)

Where width would be some number that the peaks could overlap the next category a bit.