Superscript in axis labels in ggplot2 for ions

Question

I need to achieve superscript in an axis label within ggplot2, similiar to this question: Superscript and subscript axis labels in ggplot2

However, I will need to write something like this: Ca^2+. This seems not to work with this approach, even if I put the exponent in { } with the bquote command. I tried to read through help("plotmath") but couldn't find an example for my case. I tried escaping the + with \+ and ++ but was not successful.

Edit: I would like to not use any additional packages.

Answer 1

Try to put the superscript as a literal, between quotes.

g <- ggplot(iris, aes(Species, Sepal.Length)) + geom_boxplot()
g + xlab(bquote('Superscript as a literal' ~~ Ca^'2+'))

Answer 2

Using the example you provided, this works

library(ggplot2)
qplot(uptake, data = CO2) +
  xlab(bquote('Assimilation ('*mu~ 'mol' ~Ca^{"2+"} ~ m^-2~s^-1*')'))