I found the instructions on how to plot multiple ROC curves, but I need
first to modify the sets of curves before plotting them. For instance,
suppose I have 100 ROC curves related to NaiveBayes and another 100 curves
related to J48. I want to plot the two median ROC curves for each
classifier. How can I do it? Are the ROC curves stored in a numerical format
so that I can compute the median and then plot it?