title: Greek and alpha numeric in ggplot2 axis labels
body: <p>I would like to use ggplot2 to make a chart with a axis label of uL, where the 'u' is the greek 'mu'.<br>
  to add just mu, I have found this to work</p>
  
  <pre><code>p &lt;- ggplot(data.frame(x = 1:3, y = 1:3), aes(x= x, y=y)) + geom_point()
  p + ylab(expression(mu))
  </code></pre>
  
  <p>But I have not been able to place anything else alongside it.
  These do not work</p>
  
  <pre><code>p + ylab(paste(expression(mu), "foo"))
  p + ylab("expression(mu)~foo")
  </code></pre>
  
  <p>Thanks in advance</p>
  
  <p>Sam</p>