TGraph

{
    gStyle->SetPadLeftMargin(0.15);
    gStyle->SetPadBottomMargin(0.15);
    gStyle->SetPadRightMargin(0.12);
    gStyle->SetPadTopMargin(0.08);
    gStyle->SetOptStat(0);
    //gPad.SetLogy()
    TGraph* gr = new TGraph("trydata.txt");    
    gr->SetMarkerStyle(20);
    gr->SetTitle("title");
    gr->GetXaxis()->SetTitle("energy (keV)");
    gr->GetXaxis()->CenterTitle();
    gr->GetXaxis()->SetTitleSize(0.06);
    gr->GetXaxis()->SetTitleOffset(1.1);
    gr->GetXaxis()->SetTickSize(0.05);
    gr->GetXaxis()->SetLabelSize(0.05);
    gr->GetXaxis()->SetLabelOffset(0.01);
    //gr->GetXaxis()->SetNdivisions(6,3,0);
    gr->GetXaxis()->SetRangeUser(0,8);
    gr->GetYaxis()->SetTitle("counts");
    gr->GetYaxis()->CenterTitle();
    gr->GetYaxis()->SetTitleSize(0.06);
    gr->GetYaxis()->SetTickSize(0.05);
    gr->GetYaxis()->SetLabelSize(0.05);
    gr->GetYaxis()->SetLabelOffset(0.01);
    gr->GetYaxis()->SetRangeUser(0,8);
    gr->Draw("AP");
    TGraph* gr2 = (TGraph*)gr->Clone();
    Int_t n = gr2->GetN();
    Double_t *x = gr2->GetX();
    Double_t *y = gr2->GetY();
    for(int i=0; i<n; i++)
    {
        y[i] = y[i] - 1;
    }
    gr2->SetMarkerSize(3);
    gr2->SetMarkerColor(kRed);
    gr2->SetMarkerStyle(20);
    gr2->Draw("Psame");
    Int_t xdata[6] = {1,2,3,4,5,6};
    Int_t ydata[6] = {3,6,6,1,6,3};
    TGraph* gr3 = new TGraph(6,xdata,ydata);
    gr3->SetMarkerSize(2);
    gr3->SetMarkerColor(kBlue);
    gr3->SetMarkerStyle(20);
    gr3->Draw("Psame");
}