title: Passing object to lines function ifor plot data
body: <p>My issue is the following. I use <code>ROCR</code> package to plot data.  <code>performance</code> function returns an object that I pass to plot the data like this:</p>
  
  <pre><code>example &lt;- performance(prediction1,"tpr","fpr")
  plot(example,col="red")
  </code></pre>
  
  <p>I want to add another <code>performance</code> object to this plot, but <code>lines</code> function accepts <code>x</code> and <code>y</code> coords and not an object. In fact if I do: <code>lines(example2, col="blue")</code> this error appears:</p>
  
  <blockquote>
    <p>Error in as.double(y) : 
          cannot coerce type 'S4' to vector of type 'double'**</p>
  </blockquote>