Expression in ggplot2 facet labels

Question

I would like to have an R expression in a ggplot2 facet label.

Let's say I'm plotting the tips data.frame:

library(reshape2)
> head(tips)
  total_bill  tip    sex smoker day   time size
1      16.99 1.01 Female     No Sun Dinner    2
2      10.34 1.66   Male     No Sun Dinner    3
3      21.01 3.50   Male     No Sun Dinner    3
4      23.68 3.31   Male     No Sun Dinner    2
5      24.59 3.61 Female     No Sun Dinner    4
6      25.29 4.71   Male     No Sun Dinner    4

as follows:

library(ggplot2)
sp <- ggplot(tips, aes(x=total_bill, y=tip/total_bill)) + 
         geom_point(shape=1) + 
         facet_wrap(~sex, ncol = 1)

Instead of having "Female" and "Male" as facet labels I would like to have: "Female subjects" and "Male subjects", respectively. As far as I know italicizing a label in R is achieved through the expression function, but I don't know how to combine that with facet_wrap.

Asmara

You could use a custom function, you can use with the labeller argument, see here – Disremember

It's not clear to me how you get the labeller argument of facet_grid to replace the factor with an expression and have the facets in rows with the labels on top, as in my example (rather than on the side as in facet_grid labels for rows). – Asmara

Did you check this topic ? It might help #20038343 – Oared

It's close but my labels are compound - an italicized word followed by an non-italicized one. I don't see how I can achieve that with element_text of theme – Asmara

Answer 1

Looks overly complicated, but works. You'll have to use facet_grid though.

make_label <- function(value) {
  x <- as.character(value)
  bquote(italic(.(x))~subjects)
}

plot_labeller <- function(variable, value) {
  do.call(expression, lapply(levels(value), make_label))
}

ggplot(tips, aes(x=total_bill, y=tip/total_bill)) + 
  geom_point(shape=1) + 
  facet_grid(.~sex, labeller = plot_labeller)

Jelks

I knew it had to be a function but could't get it working. Any thoughts on why this doesn't work with facet_wrap? – Disremember

@Disremember facet_wrap does not support custom labellers as is (I don't know why), but there is a low-level workaround as shown here. – Jelks

Answer 2

As of ggplot2 2.1.0:

data(tips, package="reshape2")
library(ggplot2)
ggplot(tips, aes(x=total_bill, y=tip/total_bill)) + 
  geom_point(shape=1) + 
  facet_wrap(~sex, ncol=1, 
             labeller=label_bquote(.(levels(tips$sex)[sex])~subjects))

Uvula

Fails with the error Error in levels(tips$sex)[sex] : could not find function "[" somehow the bquote seems to be evaluated in an empty environment? Not sure if this is a bug or a feature of a new ggplot2 version. – Moffit

Answer 3

The following works for R 4.0.3 and ggplot2 3.3.2

library(ggplot2)
data(tips, package="reshape2")
tips$sex = as.character(tips$sex)
sp <- ggplot(tips, aes(x=total_bill, y=tip/total_bill)) + 
  geom_point(shape=1) + 
  facet_wrap(~sex, ncol = 1, labeller = label_bquote(italic(.(sex))~subjects))
sp

It uses label_bquote as suggested by @Stéphane Laurent but does not require converting levels into the correct character within the bquote which failed for me.

Moffit

Answer 4

I know this is an old post, but I like the label_parsed function. It works with labels in a factor:

data(tips, package="reshape2")
library(ggplot2)
tips %>%
  mutate(sex = factor(sex,
                      levels = c("Female", "Male"),
                      labels = c(expression(italic(Female)~Subjects), 
                                 expression(italic(Male)~Subjects)))) %>%
  ggplot(aes(x=total_bill, y=tip/total_bill)) + 
  geom_point(shape=1) + 
  facet_wrap(~sex, ncol = 1,labeller = label_parsed)

This returns

Pellagra