Draw vertical error bars of TGraphErrors

[Viesturs]

Consider this example:

int datagr()
{
  TH1F * h = new TH1F("h", "h", 4, -2, 2);
  h -> Sumw2();
  h -> FillRandom("gaus", 500);
  //h -> Print("all");                                                          
  TGraphErrors * gr = new TGraphErrors(h);
  TCanvas * c = new TCanvas;
  gr -> Draw("AP");
  gr -> SetMarkerStyle(20);
  gr -> SetMarkerSize(1.0);
  gr -> SetLineWidth(0);
  return 0;
}

The output is:

How can I have the vertical graph error bars drawn?

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[vaubee]

If you change the line width to zero, how do you expect to see the lines from the errors? Remove that line and you will see error bars being drawn.

[Viesturs]

Then also the horizontal bars are shown. The question is about displaying the vertical error bars.

[couet]

TGraphErrors has vertical and horizontal error bars. They are defined when you create the TGraphErrors. If you do not have errors along the X axis simply define these errors to 0.

[Viesturs]

How to define the errors along X to 0?

[couet]

void datagr()
{
   TH1F * h = new TH1F("h", "h", 4, -2, 2);
   h -> Sumw2();
   h -> FillRandom("gaus", 500);
   TGraphErrors * gr = new TGraphErrors(h);
   gr -> SetMarkerStyle(20);
   gr -> SetMarkerSize(1.0);
   int n = gr->GetN();
   double ey;
   for (int i=0; i<n; i++) {
      ey = gr->GetErrorY(i);
      gr->SetPointError ( i, 0, ey);
   }
   gr -> Draw("AP");
}