Simple text labels
Simple text labels are created with element_markdown().
To demonstrate typical usage, let’s start with a basic plot of a
parabola.
library(ggplot2)
library(ggtext)
base <- ggplot(data.frame(x = c(-5, 5)), aes(x)) +
stat_function(fun = ~ .x*.x)
base
This plot would benefit from nicer axis labels. In particular, assume
we want the x axis label to read “independent variable x” and
the y axis label to read “dependent variable y =
x2”. In Markdown, we could write the axis labels as
independent variable *x* and
dependent variable *y* = *x*<sup>2</sup>.
However, if we do so, we need to tell ggplot2 to interpret the axis
labels as Markdown and not as plain text. We do this by setting
axis.title.x and axis.title.y to
element_markdown(). (Note that both are set to
element_text() in the default theme.)
base +
labs(
x = "independent variable *x*",
y = "dependent variable *y* = *x*<sup>2</sup>"
) +
theme(
axis.title.x = element_markdown(),
axis.title.y = element_markdown()
)
The new element element_markdown() behaves just like
element_text(). For example, we can modify the color or the
font size.
base +
labs(
x = "independent variable *x*",
y = "dependent variable *y* = *x*<sup>2</sup>"
) +
theme(
axis.title.x = element_markdown(color = "blue"),
axis.title.y = element_markdown(size = rel(0.8))
)
Inheritance of theme settings also works. For example, we can set
both color and font size for axis.title, and then both
axis.title.x and axis.title.y inherit the
setting.
base +
labs(
x = "independent variable *x*",
y = "dependent variable *y* = *x*<sup>2</sup>"
) +
theme(
axis.title = element_text(color = "blue", size = rel(0.8)),
axis.title.x = element_markdown(),
axis.title.y = element_markdown()
)
Note that we used element_text() instead of
element_markdown() for axis.title in the above
plot. We could have used element_markdown() as well and the
result would have been the same. It doesn’t matter that we set
axis.title = element_text(), because the
axis.title element isn’t actually rendered, only the
axis.title.x and axis.title.y elements are.
We’re setting axis.title only for the purpose of providing
shared parameter values to axis.title.x and
axis.title.y.
This is important to keep in mind when trying to create more unusual plots, e.g. with the y axis on the right. The naive code fails:
base +
labs(
x = "independent variable *x*",
y = "dependent variable *y* = *x*<sup>2</sup>"
) +
scale_y_continuous(position = "right") +
theme(
axis.title = element_text(color = "blue", size = rel(0.8)),
axis.title.x = element_markdown(),
axis.title.y = element_markdown()
)
This happens because the axis title on the right is actually drawn by
axis.title.y.right. Therefore, setting that element to
element_markdown() creates the desired result.
base +
labs(
x = "independent variable *x*",
y = "dependent variable *y* = *x*<sup>2</sup>"
) +
scale_y_continuous(position = "right") +
theme(
axis.title = element_text(color = "blue", size = rel(0.8)),
axis.title.x = element_markdown(),
axis.title.y.right = element_markdown()
)
Additional styling can be applied via inline CSS. The CSS properties
color, font-size, and font-family
are currently supported. Multi-line labels can be created by placing
<br> tags where line breaks are desired.
library(dplyr)
mtcars %>%
mutate(
transmission = ifelse(am == 1, "automatic", "manual")
) %>%
ggplot(aes(hp, mpg, color = transmission)) +
geom_point(size = 2) +
scale_color_manual(
values = c(automatic = "#0072B2", manual = "#D55E00"),
guide = "none"
) +
labs(
x = "Horse power",
y = "Miles per gallon (MPG)",
title = "<span style = 'font-size:14pt; font-family:Helvetica;'>Transmission type impacts fuel efficiency</span><br>
MPG is higher for <span style = 'color:#0072B2;'>automatic</span>
than for <span style = 'color:#D55E00;'>manual</span> transmissions"
) +
theme_bw() +
theme(
text = element_text(family = "Times"),
plot.title.position = "plot",
plot.title = element_markdown(size = 11, lineheight = 1.2)
)