title: relative position of mtext in R
body: <p>when I write text in the graph plotted by R, i use <strong>mtext</strong> command. for example, to write a index (e.g. (a),(b)  ) on the top left of a graph (inside), I can do </p>
  
  <pre><code>mtext("(c)",side=3,line=-1.5,at=0.05,cex=1.2)
  </code></pre>
  
  <p>However, the parameter <em>at</em> is the coordinates of x-axis. this is a bit annoying when the value range on x-axis is different (one has to change the <em>at</em> value for each graph). can someone give a suggestion to write text as relative values?</p>
  
  <p>Thanks in advance! </p>