Express a quantity as a function of an index

ROOT
2

Hi Mark,
in C++, how about

#include <ROOT/RDataFrame.hxx> // until v6.12, ROOT/TDataFrame.hxx

int main()
{
  TApplication app("app", nullptr, nullptr); // to keep program alive after you draw a canvas

  // until v6.12, ROOT::Experimental::TDataFrame
  ROOT::RDataFrame df("treeename", "file.root");
  auto histoy = df.Filter("x > 0").Histo1D("y");
  histoy->Draw();

  app.Run(); // keep program alive after drawing canvas
  return 0;
}

See this answer for an overview of the different available interfaces to access data stored in ROOT trees.

Cheers,
Enrico