title: plot.roc for multiclass.roc in pROC package?
body: <p>I am trying to plot multiclass ROC curves but I have not found anything fruitful in the <a href="https://cran.r-project.org/web/packages/pROC/pROC.pdf" rel="noreferrer">pROC</a> package. Here's some start code:</p>
  
  <pre><code>data(iris)
  library(randomForest)
  library(pROC)
  set.seed(1000)
  # 3-class in response variable
  rf = randomForest(Species~., data = iris, ntree = 100)
  # predict(.., type = 'prob') returns a probability matrix
  predictions &lt;- as.numeric(predict(rf, iris, type = 'response'))
  roc.multi &lt;- multiclass.roc(iris$Species, predictions)
  auc(roc.multi)
  </code></pre>
  
  <p>How do I plot the ROC curves for individual classes?</p>