title: subscript and superscript in ylab of qplot [R]
body: <p>I tried using expression(), which works with noraml plots, but my attempt below fails: any ideas?</p>
  
  <p>I want to write: µg CO2 <em>(subscript 2)</em> – C m-2 <em>(superscript -2)</em> h-1 <em>(supercript -1)</em></p>
  
  <p>works perfectly:  </p>
  
  <pre><code>plot(CO2~water_content, data=gases, ylab = expression(paste("µg ", CO[2], " - C ", m^-2, " ", h^-1, sep="")))             
  </code></pre>
  
  <p>fails:</p>
  
  <pre><code>qplot(factor(vegetation_dummy),CO2,facets=sampling~biochar,geom=c('boxplot'),data=gases_PL)+theme_bw()+xlab('Plants')+ylab = expression(paste("µg ", CO[2], " - C ", m^-2, " ", h^-1, sep=""))
  </code></pre>
  
  <p>Many thanks!</p>