title: leading superscript in plotmath expression (w/ggplot2)
body: <p>I'd like to use <code>plotmath</code> to create an axis containing a leading superscript in a <code>ggplot2</code> plot. Creating superscripts on axis labels works nicely, like so:</p>
  
  <pre><code>require(ggplot2)
  ggplot(mtcars, aes(x=disp, y=mpg)) + 
    geom_point() +
    ylab(expression(x[y]))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/qAXKG.png" alt="enter image description here"></p>
  
  <p>However, I'd like to have my axis label read "y(superscript)x" - logically that would be <code>^yx</code>, but that won't parse:</p>
  
  <pre><code>Error: unexpected '^' in: "  
  geom_point() + 
  ylab(expression(^"
  </code></pre>
  
  <p>Is there a way to force a superscript at the beginning of a statement?</p>