TGraph Range when using Log Axis

ROOT
1

Hi,

I’ve noticed when using a TGraph spanning many orders of magnitude that the zoom for the output depends on the order of setting the log-axis. If one sets the x-axis to log first the y-axis values are truncated to three orders of magnitude. While setting the y-axis to log first displays the entire range of data. This is displayed below:

LogAxisGraph.png698×475 6.66 KB

Below is the script that produced the above plot:

{
   const int numPoints = 10;
   double xValues[numPoints], yValues[numPoints];
   for (int i=0;i<numPoints;i++) {
      xValues[i] = i+2;
      yValues[i] = pow(10,i-3);
   }
   TGraph *g1 = new TGraph(numPoints, xValues, yValues);
   TGraph *g2 = new TGraph(numPoints, xValues, yValues);

   TCanvas *c = new TCanvas("c","C");
   c->DivideSquare(2);

   //Log x first
   c->cd(1);
   g1->Draw();
   gPad->SetLogx();
   c->Update();
   gPad->SetLogy();

   //Log y first
   c->cd(2);
   g2->Draw();
   gPad->SetLogy();
   c->Update();
   gPad->SetLogx();
}

This was observed with ROOT 6.08/06.

2

I see the same thing with the ROOT master. Surely a different path is taken in both cases. ROOT attempt to set the Log scale on plot having a Y value Thanks to have reported this problem. A workaround is:

   TGraph *g1 = new TGraph(numPoints, xValues, yValues);
   g1->GetHistogram()->SetMinimum(0.001);
   TGraph *g2 = new TGraph(numPoints, xValues, yValues);
   g1->GetHistogram()->SetMinimum(0.001);
3

Your workaround works as long as the minimum is greater than zero. The following also appeared to work:

g1->SetMinimum(0.001);
4

Yes … no need to specify the intermediate histogram.

If the minimum is negative the Log scale along Y will be invalid anyway and ROOT will try to find a suitable minimal value as long as the maximum is positive. If the maximum is negative also ROOT will complain that it cannot display the graph.

5

Agreed. Thanks for clarifying for other who might find this thread.

Will there be some effort to resolve the issue with the automatic determination of the limits?

6

I think this issue should be fixed. The order off setting the log scale should not have any effect on the display. Can you submit a bug report in Jira ?

7

I will give it a try.

8

https://sft.its.cern.ch/jira/secure/Dashboard.jspa
I can do it for you if you do not manage. Let me know.

9

I had no issue: https://sft.its.cern.ch/jira/browse/ROOT-8751

1 Like
10

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