title: Putting mathematical symbols and subscripts mixed with regular letters
body: <p>I want to plot a label that looks like this in <code>ggplot2</code>:</p>
  
  <p><code>Value is $\sigma$, R^{2} = 0.6</code> where <code>Value is</code> is ordinary font, <code>$\sigma$</code> is a Greek lowercase sigma letter and <code>R^{2} = 0.6</code> appears as an <code>R</code> with a superscript <code>2</code> followed by equal sign (<code>=</code>) followed by <code>0.6</code>. How can this be used in ggplot <code>factor</code>s and in arguments to things like <code>xlab,ylab</code> of R?</p>