How to italicize one category in a legend in ggplot2

Question

For my legend in ggplot2 I have 2 categories, how do I italicize only 1 category and not the other?

As an example, consider the following plot. How can I set \"Manu

Answer 1

You can use expression and italic to create italicized text on labels.

library(ggplot2)

ggplot(data = mtcars, aes(x = as.factor(am), fill = as.factor(am))) + 
  geom_bar() + 
  scale_fill_discrete(
    "Transmission",
    breaks = c(0, 1),
    labels = c("Automatic", expression(italic("Manual")))
  )

^{Created on 2020-01-01 by the reprex package (v0.3.0)}

Answer 2

I have been working on a simpler and more flexible way to achieve this goal, via the ggtext package that enables markdown styling in ggplot. This is currently under development but should be released to CRAN soon (early 2020).

library(ggplot2) # may require: remotes::install_github("tidyverse/ggplot2")
library(ggtext)  # remotes::install_github("clauswilke/ggtext")

ggplot(data = mtcars, aes(x = as.factor(am), fill = as.factor(am))) + 
  geom_bar() + 
  scale_fill_discrete(
    "Transmission",
    breaks = c(0, 1),
    labels = c("Automatic", "*Manual*")
  ) +
  theme(legend.text = element_markdown())

^{Created on 2020-01-01 by the reprex package (v0.3.0)}