expanding mlr's visualization and exploratory data analysis capabiliites

by Zachary Jones for R Project for Statistical Computing

I will expand comparison plots of learners, add interactive visualization via ggvis that duplicates all existing plotting functionality, and add methods to visualize performance on subsets of the feature space. I will add partial and marginal dependence for all supervised learners, associated plotting methods, and feature importance measures. This will be done in C++ via Rcpp where possible.