TH1D *hist = 0;

double model(double x, double *par)
{
    double mu    = par[3];
    double sigma = par[4];
    double norm  = 1./sqrt(2.*TMath::Pi())/sigma;
    double G     = norm*exp(-0.5 *pow((x-mu)/sigma,2));
    double BinWidth = hist->GetBinWidth(1);
    
    return par[0] + par[1]*x + par[2] * BinWidth * G;
}

void fcn(int &npar, double *gin, double &f, double *par, int iflag)
{
    f = 0.;
    for(int i=1;i<=hist->GetNbinsX();i++) {
        double x       = hist->GetBinCenter(i);
        double measure = hist->GetBinContent(i);
        double error   = sqrt(measure);
        double func    = model(x,par);
        double delta   = (func - measure)/error;
        
        f += delta*delta;
    }
}

void example_05()
{
    TFile *fin = new TFile("example_data.root");
    hist = (TH1D *)fin->Get("hist");
    
    hist->SetFillColor(kRed-9);
    hist->SetStats(false);
    hist->GetXaxis()->SetTitle("Mass");
    
    TMinuit *gMinuit = new TMinuit(5);
    gMinuit->SetFCN(fcn);
    
    gMinuit->DefineParameter(0, "p0",    100., 1.,    0.,  200.);
    gMinuit->DefineParameter(1, "p1",    -30., 1., -200.,  200.);
    gMinuit->DefineParameter(2, "area", 2000., 1.,    0.,20000.);
    gMinuit->DefineParameter(3, "mean",  1.00, 1.,   0.5,   1.5);
    gMinuit->DefineParameter(4, "width", 0.05, 1., 0.001,  0.15);
    
    gMinuit->Command("MIGRAD");
    gMinuit->Command("MIGRAD");
    gMinuit->Command("MINOS");
    
    double par[5],err[5];
    for(int i=0;i<5;i++) gMinuit->GetParameter(i,par[i],err[i]);
    
    TH1F* curve = new TH1F("curve","curve",hist->GetNbinsX()*5,
                           hist->GetXaxis()->GetXmin(),hist->GetXaxis()->GetXmax());
    
    for(int i=1;i<=curve->GetNbinsX();i++) {
        double x = curve->GetBinCenter(i);
        double f = model(x,par);
        curve->SetBinContent(i,f);
    }
    curve->SetLineWidth(3);
    
    hist->Draw();
    curve->Draw("csame");
}