title: force R plot through origin
body: <p>I have a simple plot in R:</p>
  
  <pre><code>roc = plot(RP.perf@alpha.values[[1]],RP.perf@x.values[[1]],type="l",ylim=c(0,1));
  </code></pre>
  
  <p>however would like to force the graph through the origin. is there anyway to do this? I can't find an option for this in the plot function.</p>
  
  <p>Thanks.</p>