void exampleParamScan()  {

   auto h1 = new TH1D("h1","h1",100,-5,5);
   h1->FillRandom("gaus");

   auto result = h1->Fit("gaus","LS");

   // make scan of likelihood function

   // create graph with 100 points 
   auto graph = new TGraph(100);

   auto fitter = (TBackCompFitter*) TVirtualFitter::GetFitter();

   // scan the parameter 2 (sigma) in a range 10* the parameter error
   double pmin = result->Parameter(2) - 10*result->Error(2); 
   double pmax = result->Parameter(2) + 10*result->Error(2); 
   fitter->Scan(2, graph, pmin,pmax);

   graph->Draw("AC");
   
}