Producing two lines of complex y axis titles using atop, \n, etc in ggplot in R

Question

I am trying to produce two lines in a y axis label in ggplot in R, where the second line has some relatively complex text that seems to make it difficult to print on two lines.

On a single line, I have gotten this script to print just fine using bquote:

Figure 1:

ggplot(data = iris, aes(x = Sepal.Width, y = Sepal.Length)) + geom_point() + 
  ylab(bquote('Efflux ('*mu~ 'mol' ~CO[2]~ m^-2~s^-1*')')) 

And, with relatively simple text, I can use the \n feature within ylab to produce two lines:

Figure 2:

ggplot(data = iris, aes(x = Sepal.Width, y = Sepal.Length)) + geom_point() + 
  ylab("Efflux \n Carbon Dioxide")

However, trying to use the atop feature within bquote, I have struggled to figure out how to use commas, quotations marks etc. to properly print these two lines of y axis labels:

ggplot(data = iris, aes(x = Sepal.Width, y = Sepal.Length)) + geom_point() + 
  ylab(bquote(atop("Efflux", ('*mu~ 'mol' ~CO[2]~ m^-2~s^-1*'))))

This prints an error, which I know has to do with placement of quotation marks. Does anyone have a lead on how to print this more complex text over two lines?

Answer 1

You could try using element_markdown from the ggtext package. This allows you to use markdown (or html) to produce the line breaks, symbols, subscripts and superscripts you need without resorting to bquote , which struggles with multi-line inputs. You can even show the units in italics, as in this example:

library(ggplot2)
library(ggtext)

ggplot(data = iris, aes(x = Sepal.Width, y = Sepal.Length)) + 
  geom_point() + 
  ylab("Efflux<br><i>(&mu;mol CO<sub>2</sub> m<sup>-2</sup>s<sup>-1</sup>)</i>") +
  theme(axis.title.y = element_markdown())