title: Tick mark text direction in R
body: <p>I am probably overlooking something obvious but then maybe someone else will too.</p>
  
  <p>I'm using a lot of ticks on the x-axis in an R plot.
  I have shrunk the margin text to make room (using <code>cex.axis</code>) but I want more and it would be better if I could make the text vertical.</p>