X-axis labels with TGraph

ROOT
1

Hello

I have TGraph with N data points and to each data point I’d like have a corresponding string as label on the X-axis. I tried to use this example:

{
#include <sstream>;

  Int_t numchains=3;

  Double_t chain[numchains];
  Double_t chainerr[numchains];

  for (Int_t i=0;i<numchains;i++){
    chain[i]=i;
    chainerr[i]=0.5;
  };

  Double_t time[numchains] = {
  0.32528,
  2.48328,
  0.3505
  };

  Double_t l2timerms[numchains] = {
  0.305687,
  12.0619,
  0.189249
  };

  std::string names[numchains] = {
  "sequence_L2_muon_standalone_mu6l",
  "sequence_L2_mu6l",
  "sequence_L2_te650"
  };

  TGraphErrors *l2timevschain = new TGraphErrors(numchains,chain,time,chainerr,l2timerms);

  for(Int_t k=0;k<numchains;k++){
    l2timevschain->GetHistogram()->GetXaxis()->SetBinLabel(k,names[k].c_str());
  };

  l2timevschain->SetMarkerStyle(21);
  l2timevschain->Draw("AP");
}

but it gives me a graph where the X-axis labels are all on the left edge of the graph and do not coincide with the data points. How can I fix this?

thanks
Ignacio

2

The labels are set to the bins of the underlaying histogram. By default this histogram has 100 bins so if you set the labels to the first 3 bins you get what you described. I see two possible ways:

  1. You set the labels to the bins numbers corresponding to the points positions.

  2. You reduce the bins number of the underlaying histogram to 3. The following example shows that solution.

{
   TCanvas *c1 = new TCanvas("c1", "c1",15,49,1051,500);   

   Int_t numchains=3;

   Double_t chain[numchains];
   Double_t chainerr[numchains];

   for (Int_t i=0;i<numchains;i++){
   chain[i]=i;
   chainerr[i]=0.5;
   };

   Double_t time[numchains] = {
   0.32528,
   2.48328,
   0.3505
   };

   Double_t l2timerms[numchains] = {
   0.305687,
   12.0619,
   0.189249
   };

   std::string names[numchains] = {
   "sequence_L2_muon_standalone_mu6l",
   "sequence_L2_mu6l",
   "sequence_L2_te650"
   };

   TGraphErrors *l2timevschain = new TGraphErrors(numchains,chain,time,chainerr,l2timerms); 
   TAxis *ax = l2timevschain->GetHistogram()->GetXaxis();
   Double_t x1 = ax->GetBinLowEdge(1); 
   Double_t x2 = ax->GetBinUpEdge(ax->GetNbins());
   l2timevschain->GetHistogram()->GetXaxis()->Set(3,x1,x2);

   for(Int_t k=0;k<numchains;k++){
   l2timevschain->GetHistogram()->GetXaxis()->SetBinLabel(k+1,names[k].c_str());   
   } 

   l2timevschain->SetMarkerStyle(21);
   l2timevschain->Draw("AP");
}
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