I'm confusing about width(height) argument in geom_jitter()

[kimchpekr]

"width(height)" argument in geom_jitter() is described as belows.

width

Amount of vertical and horizontal jitter. The jitter is added in both positive and negative directions, so the total spread is twice the value specified here. If omitted, defaults to 40% of the resolution of the data: this means the jitter values will occupy 80% of the implied bins.

So, I expected geom_jitter() and geom_jitter(width = 0.4) will produce same plot and "width = 0.4" spreads points across a range "base ± resolution * 0.4". but the results are not.

> p <- ggplot(mtcars, aes(gear, mpg))
> p + geom_jitter()  ## jitter range is 3 ± 0.4

> p + geom_jitter(width = 0.4) ## jitter range is narrow than 3 ± 0.4

I'm confusing about width argument and how can i calculate a jitter range.

I expect like belows but the result doesn't

geom_jitter(width = 0.2) will spread points across a range [2.8 ~ 3.2] : 3 ± 0.2
geom_jitter(width = 0.4) will spread points across a range [2.6 ~ 3.4] : 3 ± 0.4
geom_jitter(width = 0.6) will spread points across a range [2.4 ~ 3.6] : 3 ± 0.6

but, geom_point(aes(x = jitter(gear, amount = 0.2)) produce a plot what i expected.

Please, give me some hints...

[has2k1]

That is clearly a bug. I think the width is being treated as a substitute for the resolution, if that is the case then the effective amount is 0.4 * 0.4 = 0.16

[kimchpekr]

@has2k1 Thanks for comment.

data resolution at "gear = 3" is 1, so amount should be calculated 1 * 0.4 = 0.4 instead of 0.4 * 0.4 = 0.16

> resolution(mtcars$gear[mtcars$gear == 3])
[1] 1

but you right.
geom_jitter(width = 0.4) spreads points across a range you calculated 3±0.16 like below.

I'm still confusing....

[has2k1]

I've since looked at the code and I'll submit a PR that fixes it.

[kimchpekr]

@has2k1
I'm looking forward to it.

[svenhalvorson]

I'm having a hard time knowing what this means:
'If omitted, defaults to 40% of the resolution of the data: this means the jitter values will occupy 80% of the implied bins.'

You're saying that the range of values that point could be mapped to is 3 +/- (resolution) * 0.4 so how is resolution determined?

[has2k1]

http://ggplot2.tidyverse.org/reference/resolution.html