title: Subscript letters in ggplot axis label
body: <p>I'm trying to work out how to have subscript letters in an axis label. </p>
  
  <pre><code>dat &lt;- data.frame(x = rnorm(100), y = rnorm(100))
  ggplot(dat, aes(x=x,y=y)) +
      geom_point() +
      labs(y=expression(Blah[1]))
  
  dat &lt;- data.frame(x = rnorm(100), y = rnorm(100))
  ggplot(dat, aes(x=x,y=y)) +
      geom_point() +
      labs(y=expression(Blah[1d]))
  </code></pre>
  
  <p>The first example works as it's just a number, as soon as you have a character in the square brackets, it fails. Blah[subscript(1d)] is essentially what I need, but I can't work out how to get it to let me have letters in subscript. I have tried variations, including <code>paste()</code>.</p>
  
  <p>The following examples provide strange behavior:</p>
  
  <pre><code>labs(y=expression(Blah[12])) # this works
  labs(y=expression(Blah[d])) # this works
  labs(y=expression(Blah[d1])) # this works
  labs(y=expression(Blah[1d])) # this fails
  </code></pre>
  
  <p>Thoughts?</p>