title: Reducing spacing between lines when using atop
body: <p>I have been using <code>atop</code> and <code>expression</code> in R to create axis labels for some figures, as I need to italicise some words, use superscripts and my labels need to be over two lines.</p>
  
  <p>This is an example of what I'm using:</p>
  
  <pre><code>plot(c(1:10),ylab="")
  mtext(side=2,line=2,expression(atop('Mean height of ', italic(Dicksonia~antarctica)*  ' (m)')))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/1e4jA.png" rel="noreferrer"><img src="https://i.stack.imgur.com/1e4jA.png" alt="enter image description here"></a></p>
  
  <p>However, the space between the two lines is too big for my figures. Can I reduce this space?</p>