Rotating text of secondary axis labels

Question

I'm taking advantage of the recently added feature of secondary axis labels in ggplot2. I'd like to rotate just the secondary axis but have been unable to find documentation or work out how to do this.

Its simple enough to rotate all text using...

ggplot(mtcars, aes(x = wt, y = mpg, colour = mpg)) +
    geom_point() +
    scale_x_continuous(name = 'Bottom Axis',
                       sec.axis = sec_axis(trans = ~ .,
                                           name  = 'Top Axis',
                                           breaks = c(2:5),
                                           labels = c('Two Two', 'Three Three Three', 'Four Four Four Four', 'Five Five Five Five Five'))) +
## Rotate text of x-axis
    theme(axis.text.x = element_text(angle = 90))

Its not mentioned in any of the documentation I've read (e.g. scale_continuous and themes) how to achieve rotating of just one axis.

My motivation for requiring this is that some of the labels I wish to apply to my data are long and overlap when placed horizontally, by rotating them I can avoid this, but I wish to retain the horizontal orientation on the bottom axis.

Answer 1

As long as you have a relatively current version of ggplot2, you can use axis.text.x.top:

ggplot(mtcars, aes(x = wt, y = mpg, colour = mpg)) +
  geom_point() +
  scale_x_continuous(
    name = 'Bottom Axis',
    sec.axis = sec_axis(
      trans = ~ .,
      name  = 'Top Axis',
      breaks = 2:5,
      labels = c('Two Two', 'Three Three Three', 'Four Four Four Four', 'Five Five Five Five Five')
    )
  ) +
  ## Rotate text of x-axis
  theme(axis.text.x.top = element_text(angle = 45, hjust = 0))