From: CERN: 2014 Google Summer of Code students present their projects

Omar Zapata, a Computer Science student from Medellin, Colombia, worked on developing an interface between ROOT and R, a software framework for statistical computing. With his work it is now possible to use the very large set of statistical tools present in R inside ROOT and with data stored in ROOT objects. These tools which include some of the latest algorithms developed by the statistics and computing research community. Omar wrote classes which perform the translation from the ROOT objects to R objects in order to call R functions and to return the result to ROOT in order to visualize or store it. His work is available as a public, dedicated branch of ROOT in github; in the near future it will be migrated in the ROOT official report.

UPDATE: ROOT-R is now in the ofcial master of ROOT From Omar Zapata: include ROOT-R interface allowing to use R functions inside ROOT.