void fit()
{
  gStyle->SetOptFit();
  const int kN = 100;
  const double kScale = 1e-1;

  TGraphErrors* gra[2];
  for(int i = 0; i < 2; i++){
    gra[i] = new TGraphErrors(kN);
  } // i

  gra[0].SetTitle(";x;y + #deltay;");
  gra[1].SetTitle(";y + #deltay;x;");

  for(int i = 0; i < kN; i++){
    double ran = gRandom->Gaus();
    double x = i*kScale;
    double y = (i + ran)*kScale;
    double e = kScale;
    gra[0]->SetPoint(i, x, y);
    gra[1]->SetPoint(i, y, x); // change x<=>y
    gra[0]->SetPointError(i, 0, e);
    gra[1]->SetPointError(i, e, 0); // change ex<=>ey
  } // i

  TCanvas* can = new TCanvas("can", "can");
  can->Divide(2, 1);

  for(int i = 0; i < 2; i++){
    can->cd(i + 1);
    gra[i]->Draw("ap");
    gra[i]->Fit("pol1");
  } // i

}