Make sure jitter is only calculated once

[thomasp85]

Fix #2941

This PR changes the jitter position implementation to calculate a jitter once for x and y and then apply that to all position aesthetics

[clauswilke]

I'm not entirely sure yet this works in all edge cases. I believe jitter() will look at the data to consider how much jitter to apply. This gets overwritten with the amount argument, but if amount is NULL or 0 then jitter() again looks at the data. At a minimum this requires a warning in the release notes, I believe.

A safer approach might be to use some actual x or y data columns as dummy data, if available, and then subtract them from the dummy data before applying the jitter. Get's very circuitous, though.

[clauswilke]

Looks like a long time ago I came up with a different way to solve this issue. See if you like this approach:
https://github.com/clauswilke/ggplot2/blob/ed9be70c14e67e5448ac9f11aadce2cfb868764d/R/position-jitter.r#L75-L91

[thomasp85]

Yeah, I saw that. I still prefer to calculate one jitter value per dimension and apply that. You are right that we need to extract real values for it but that is trivial

[clauswilke]

Looks good, but I would flip the order in the intersect() command to get x if available.

intersect(c("xmin", "x", "y"), c("x", "xmin"))[1]
#> [1] "xmin"
intersect(c("x", "xmin"), c("xmin", "x", "y"))[1]
#> [1] "x"

^{Created on 2021-04-09 by the reprex package (v1.0.0)}

[thomasp85]

Ah - good catch... I thought I was doing the right way🙈