ROOT::RDataFrame r(256);
auto rr = r.Define("v", [](ULong64_t e){return e;}, {"rdfentry_"})
           .Define("w", [](ULong64_t e){return 1./(e+1);}, {"v"});

auto stats_eu = rr.Stats<ULong64_t>("v");
auto stats_ew = rr.Stats<ULong64_t, double>("v", "w");
auto stats_iu = rr.Stats("v");
auto stats_iw = rr.Stats("v", "w");

stats_eu->Print();
stats_ew->Print();
stats_iu->Print();
stats_iw->Print();

gROOT->GetListOfCanvases()->Draw()