leading superscript in plotmath expression (w/ggplot2)

Question

I'd like to use plotmath to create an axis containing a leading superscript in a ggplot2 plot. Creating superscripts on axis labels works nicely, like so:

require(ggplot2)
ggplot(mtcars, aes(x=disp, y=mpg)) + 
  geom_point() +
  ylab(expression(x[y]))

However, I'd like to have my axis label read "y(superscript)x" - logically that would be ^yx, but that won't parse:

Error: unexpected '^' in: "  
geom_point() + 
ylab(expression(^"

Is there a way to force a superscript at the beginning of a statement?

Answer 1

How about this:

ggplot(mtcars, aes(x=disp, y=mpg)) + 
  geom_point() +
  ylab(expression(phantom(0)^y * x))

I was certain there must be a way to do a "placeholder" character, but I had to scroll down a bit in ?plotmath to find it.