Retrieving an unnamed TGraph

ROOT
9

Try:

{
  TFile *f = TFile::Open("test.root");
  // f->ls();
  TGraph *o;
  TMultiGraph *mg = new TMultiGraph();
  TString name = ""; // usually "Graph" or ""
  for (Int_t i = 1; i < 9999 ; i++) { // 9999 = a "memory object"
#if 1 /* 0 or 1 */
    // checks that the object with the given "name" is a TGraph
    f->GetObject(name + ";" + ((Long_t)i), o);
#else /* 0 or 1 */
    // assumes that any object with the given "name" is a TGraph
    o = ((TGraph *)(f->Get(name + ";" + ((Long_t)i))));
#endif /* 0 or 1 */
    // if (o) f->Remove(o); // needed for TH* and similar objects
    if (!o) break; // no more TGraph found
    mg->Add(o);
  }
  if (mg->GetListOfGraphs()) mg->Draw("A");
}

BTW. If you need to retrieve, e.g., TGraphAsymmErrors objects, replace all occurrences of TGraph in this macro with TGraphAsymmErrors.