Revision 754

Add new class TMultiGraph.
//   A TMultiGraph is a collection of TGraph (or derived) objects
//   Use TMultiGraph::Add to add a new graph to the list.
//   The TMultiGraph owns the objects in the list.
//   Drawing options are the same as for TGraph
//   Example;
//     TGraph *gr1 = new TGraph(...
//     TGraphErrors *gr2 = new TGraphErrors(...
//     TMultiGraph *mg = new TMultiGraph();
//     mg->Add(gr1);
//     mg->Add(gr2);
//     mg->Draw("alp");