Using ggplot2 3.5.0, I have found that the line ends of error bars do not stay centered when using position_jjitterdodge, although it works properly with position_dodge. This also leads to unexpected behavior when the error bar width is set to 0 such that little nubbins of line ends of varying width appear some distance from the rest of the error bar.

First, an example that works as expected with position_dodge:

library(tidyverse)
data = tibble(
  x = factor(c('a', 'a', 'a', 'a', 'b', 'b', 'b', 'b')),
  group = factor(c('c', 'd', 'c', 'd', 'c', 'd', 'c', 'd')),
  lower = seq(0, 3, length.out = 8),
  middle = seq(1, 4, length.out = 8),
  upper = seq(2, 5, length.out = 8)
)

# looks fine
ggplot(data, aes(x = x, color = group)) +
  geom_errorbar(aes(ymin = lower, ymax = upper),
                position = position_dodge(width = 0.75))

Second, an example that results in error bar ends that are not centered and have variable width (unexpected):

It is unclear to me where exactly the line ends are centered. My initial guess that they might be centered using the dodge.width, doesn't appear to be true by looking at the data located at x = a, group = c.

# ends of errorbars are not centered with postion_jitterdodge
ggplot(data, aes(x = x, y = middle, color = group)) +
  geom_errorbar(aes(ymin = lower, ymax = upper),
                position = position_jitterdodge(
                  dodge.width = 0.75,
                  seed = 1
                )
  )

Third, the unexpected behavior persists when width = 0:

The remaining nubbins when width = 0 appear to be related to the differing widths shown in the last example.

# setting width to 0 still shows the same issue, but now the error bars are little nubbins floating off in space
ggplot(data, aes(x = x, y = middle, color = group)) +
  geom_errorbar(aes(ymin = lower, ymax = upper),
                width = 0,
                position = position_jitterdodge(
                  dodge.width = 0.75,
                  seed = 1
                )
  )

Related error discovered while writing this reprex:

(this is not really the main issue, and I'm happy to move it to a different issue, if that is preferred)

position_jitterdodge requires the aesthetic, y, even though geom_errorbar does not. I'm not sure if this is the expected behavior, but it seemed unexpected from my vantage point of only wanting a horizontal jitterdodge.

# does not work without y, even though geom_errorbar doesn't require y
ggplot(data, aes(x = x, color = group)) +
  geom_errorbar(aes(ymin = lower, ymax = upper),
                position = position_jitterdodge(
                  dodge.width = 0.75,
                  seed = 1
                )
  )

Error in geom_errorbar(aes(ymin = lower, ymax = upper), position = position_jitterdodge(dodge.width = 0.75, :

ℹ Error occurred in the 1st layer.
Caused by error in `setup_data()`:
! `position_jitterdodge()` requires the following
missing aesthetics: y.