How to set expressions as axis text of facets in ggplot2?

Question

I am trying to set expressions as x-axis text in facet environment in ggplot2 with unequal length of labels. For example:

dat <- structure(list(Species = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 2L), .Label = c("A", "B"), class = "factor"), Individual = structure(c(1L, 
2L, 3L, 5L, 1L, 2L, 3L, 4L, 5L), .Label = c("1", "2", "3", "4", 
"expression(bar(\"x\"))"), class = "factor"), mean = c(45, 32, 
100, 59, 65, 110, 87, 93, 88.75), min = c(34, 20, 89, 47.66666667, 
54, 100, 67, 85, 76.5), max = c(54, 42, 110, 68.66666667, 76, 
120, 99, 105, 100)), .Names = c("Species", "Individual", "mean", 
"min", "max"), class = "data.frame", row.names = c(NA, -9L))

require(ggplot2)

This answer describes how to do it without facets. I manage set the labels, but I cannot find how to set labels for each facet separately:

ggplot(dat, aes(x = Individual, y = mean, ymin = min, ymax = max, color = Species)) + 
  geom_pointrange() + 
  facet_wrap(~Species, scales = "free") + 
  scale_x_discrete(labels = c("1", "2", "3", expression(bar("x")))) + 
  theme_grey(base_size = 18)

A long vector or a list specifying labels does not seem to work:

ggplot(dat, aes(x = Individual, y = mean, ymin = min, ymax = max, color = Species)) + 
  geom_pointrange() + 
  facet_wrap(~Species, scales = "free") + 
  scale_x_discrete(labels = c("1", "2", "3", expression(bar("x")), "1", "2", "3", "4", expression(bar("x")))) +
  theme_grey(base_size = 18)

ggplot(dat, aes(x = Individual, y = mean, ymin = min, ymax = max, color = Species)) + 
  geom_pointrange() + 
  facet_wrap(~Species, scales = "free") + 
  scale_x_discrete(labels = list(c("1", "2", "3", expression(bar("x")), c("1", "2", "3", "4", expression(bar("x")))))) + 
  theme_grey(base_size = 18)

Is there a way doing this in ggplot2?

Luca

In this example they are different species and expression(bar("x")) is the mean value over individuals (1:4 in facet B). – Luca

Answer 1

You can add the breaks arguments to scale_x_discrete:

ggplot(dat, aes(x=Individual, y = mean,ymin = min, ymax = max, color=Species)) + 
geom_pointrange() + facet_wrap(~Species, scales = "free_x") +
scale_x_discrete(breaks=levels(factor(dat$Individual)),
               labels = c("1", "2", "3", "4", expression(bar("x")))) +
theme_grey(base_size = 18)

Elva

Answer 2

As a workaround, you can use scales = "free_y":

ggplot(dat, aes(x = Individual, y = mean, 
                ymin = min, ymax = max, color = Species)) + 
  geom_pointrange() + facet_wrap(~Species, scales = "free_y") +
  scale_x_discrete(labels = c("1", "2", "3", "4", expression(bar("x")))) +
  theme_grey(base_size = 18)

Caesar