title: Expression and new line in plot labels
body: <p>I want to add some subscripts and superscripts to my graph labels. I've try <code>expression</code>, but it doesn't work as I wish with new lines (<code>\n</code>). I've try to fix it using <code>paste</code>, but it doesn't work. Below are some of my tries:</p>
  
  <pre><code>par(mfcol=c(1,3))
  plot(1,1,main=expression("first line \n second line x"^2))
  plot(1,1,main=expression(paste("first line \n second line", "x"^2)))
  plot(1,1,main=paste("first line \n second line", expression("x"^2)))
  </code></pre>
  
  <p>It produces:</p>
  
  <p><img src="https://i.stack.imgur.com/PghZd.jpg" alt="enter image description here"></p>
  
  <p>In first two pictures the second line is not well centered, in the third one the superscript fails. How to get both centered line and subscripts/superscripts?</p>