TVirtualFitter::SetDefaultFitter("Minuit");

TCanvas *c1 = new TCanvas("c1");
TH1F *h = new TH1F("h","My histogram",100,-3,3);
h->FillRandom("gaus",6000);
h->Fit("gaus");
c1->Update();

TCanvas *c2 = new TCanvas("c2","contours",10,10,600,800);
c2->Divide(1,2);
c2->cd(1);
/*get first contour for parameter 1 versus parameter 2*/
TGraph *gr12 = (TGraph*)gMinuit->Contour(40,1,2);
gr12->Draw("alp");
c2->cd(2);
/*Get contour for parameter 0 versus parameter 2  for ERRDEF=2*/
gMinuit->SetErrorDef(4); //note 4 and not 2!
TGraph *gr2 = (TGraph*)gMinuit->Contour(80,0,2);
gr2->SetFillColor(42);
gr2->Draw("alf");
/*Get contour for parameter 0 versus parameter 2 for ERRDEF=1*/
gMinuit->SetErrorDef(1);
TGraph *gr1 = (TGraph*)gMinuit->Contour(80,0,2);
gr1->SetFillColor(38);
gr1->Draw("lf");

gROOT->GetListOfCanvases()->Draw()