Hi Marco,
See comments in TTree::draw at URL:
http://root.cern.ch/root/html/src/TTree.cxx.html#TTree:Draw
This includes the following section:
//
// How to obtain more info from TTree::Draw
// ========================================
//
// Once TTree::Draw has been called, it is possible to access useful
// information still stored in the TTree object via the following
functions:
// -GetSelectedRows() // return the number of entries accepted by
the
// //selection expression. In case where no
selection
// //was specified, returns the number of
entries processed.
// -GetV1() //returns a pointer to the float array of V1
// -GetV2() //returns a pointer to the float array of V2
// -GetV3() //returns a pointer to the float array of V3
// -GetW() //returns a pointer to the double array of
Weights
// //where weight equal the result of the
selection expression.
// where V1,V2,V3 correspond to the expressions in
// TTree::Draw("V1:V2:V3",selection);
//
// Example:
// Root > ntuple->Draw("py:px","pz>4");
// Root > TGraph *gr = new TGraph(ntuple->GetSelectedRows(),
// ntuple->GetV2(), ntuple->GetV1());
// Root > gr->Draw("ap"); //draw graph in current pad
// creates a TGraph object with a number of points corresponding to
the
// number of entries selected by the expression "pz>4", the x points
of the graph
// being the px values of the Tree and the y points the py values.
Rene Brun