Things to do
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- Add position_points_beeswarm() and position_points_quasirandom()
- Add stat for categorical data, as described here: https://stackoverflow.com/a/47614529/4975218
- Add more visual tests, and figure out why vdiffr has stopped working.
- In legend drawing code, can we detect the presence of jittered points from the input data, not from the parameter setting?

