title: Multiple ROC curves in one plot ROCR
body: <p>Is it possible to plot the roc curve for diffrent classifiers in the same plot using the ROCR package? I've tried:</p>
  <pre><code>&gt;plot(perf.neuralNet, colorize=TRUE)
  &gt;lines(perf.randomForest)
  </code></pre>
  <p>But I get:</p>
  <blockquote>
  <p>Error en as.double(y) :
  cannot coerce type 'S4' to vector of type 'double'</p>
  </blockquote>
  <p>Thank you!</p>