TGraph, point in the origin

ROOT
1

Dear all,

When I’m plotting an empty graph, I get a strange point in the origin plotted. How can I get rid of it? I’m using 5.20 on Linux, CINT/ROOT C/C++ Interpreter version 5.16.29.

A sample code is below:

// ROOT files
#include "TROOT.h"
#include "TCanvas.h"
#include "TGraph.h"
#include "TAxis.h"

int main()
{
  const Int_t npts=10;

  TCanvas *c_graph = new TCanvas("c_graph","");
  TGraph  *g_graph = new TGraph(npts);

  g_graph->SetMarkerStyle(22);
  g_graph->SetMarkerSize(1.6);
  g_graph->SetMarkerColor(kRed);

  g_graph->GetXaxis()->SetLimits(-1.0,1.0);

  c_graph->cd(); g_graph->Draw("AP");

  c_graph->Print("graph_test.png");
}

The output is attached.
Thank you,
Slava

graph_test.png692×444 2.5 KB

2

When doing: TGraph *g_graph = new TGraph(npts); npts points are allocated and the x[i],y[i] set to 0, hence your result.
I do not understand what you try to achieve in drawing a graph without defining the points. If you simply want to set the pad scale you should call pad.DrawFrame(xmin,ymin,xmax,ymax).

Rene

3

[quote=“brun”]When doing: TGraph *g_graph = new TGraph(npts); npts points are allocated and the x[i],y[i] set to 0, hence your result.
I do not understand what you try to achieve in drawing a graph without defining the points. If you simply want to set the pad scale you should call pad.DrawFrame(xmin,ymin,xmax,ymax).

Rene[/quote]

This is a simplification of a real life problem when there are situations when some/all point of a graph are not filled in a loop.

So, are you saying that whenever I don’t set some of the points from the number given in the constructor they will be defaulted to 0, consequently drawn at (0,0) and nothing can be done about that?

Slava

4

Yes, of course, the points must be defined before drawing the graph.
Why dont’ you define your graph with only the points that you can set.
You can always add new points to a TGraph.

Rene

1 Like
5

[quote=“brun”]Yes, of course, the points must be defined before drawing the graph.
Why dont’ you define your graph with only the points that you can set.
You can always add new points to a TGraph.

Rene[/quote]

Yes, I could use the default constructor and add only necessary points. Just I was not aware I could add them. After your reply I looked at TGraph in the reference guide again, but the only thing referring to adding points I could find was InsertPoint: Insert a new point at the mouse position This would not help me in the batch mode.

Slava

6

root.cern.ch/root/html/TGraph.ht … h:SetPoint
root.cern.ch/root/html/src/TGrap … tml#WjpSHE
root.cern.ch/root/html/TGraph.ht … emovePoint

etc