TGraph2D - palette underestimates values given on z axis

[Richard_B]

Hello,

The palette displays a shifted scale on the values on the z axis on a TGraph2D. It shows as if the ploted values were smaller than what I entered. I tried using a TH2 object, and the values are matching the palette scale, but using a TGraph2D is more appropriate for my work. I did tests on a simplified code. I show below the output that I rotated to display the z-axis in parallel with the palette scale.

I also tried to increase the default “NunberContours” – same issue.

I would be grateful if someone can tell me how to correct this shift in the palette scale.

Cheers,

Richard.

_ROOT Version: 6.24/06
_Platform: Ubuntu 20.04.4 LTS
Compiler: gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

{
   const Int_t N = 10;

   Float_t x[N]= {1. ,  2. ,  3. , 4. ,  5. ,  6.  , 7.   ,  8.,  9. , 10.  };
   Float_t y[N]= {10., 12. , 11. , 8. , 10. , 10.  , 12.  , 11.,  8. , 10.  };
   Float_t z[N]= {0.9,  0.2,  0.4, 0.7,  0.6,  0.95,  0.33,  1.0, 0.5,  0.99};
   
   gStyle->SetPalette(1);

   TGraph2D *g=new TGraph2D(N,x,y,z);
   g->SetMarkerStyle(kFullCircle);
   g->SetMarkerSize(2);
   g->Draw("PCOLZ");
   gPad->SetGridx(); // not working 
   gPad->SetGridy(); // not working 
   //g->SetMinimum(0.2);
   //g->SetMaximum(1.0);
   gPad->SetTheta(90.); gPad->SetPhi(0.001);
   g->GetXaxis()->SetTitle("x axis");
   g->GetYaxis()->SetTitle("y axis");
   g->GetZaxis()->SetTitle("z axis");

   c1->Print("Test_Graph_2D.png");
 }

[Dilicus]

Hi and welcome to the root forum.

I used gStyle->SetNumberContours(160);
The results seems fine (have a look to the figure attached):

I do not know precisely how the colors are assigned to each marker, maybe there is some rounding issue.

Stefano

[Richard_B]

Hi Stefano,
Thank you for your feedback. Using this value of 160 is not perfect for me (some difference with your figure) but it makes my plot acceptable now.
Regards,
Richard.

[dastudillo]

If you only use g->SetMaximum(1.0); without SetMinimum, it looks better:

{
  TCanvas *c1 = new TCanvas();
  const Int_t N = 10;

  Float_t x[N]= {1. , 2. , 3. , 4. , 5. , 6. , 7. , 8., 9. , 10. };
  Float_t y[N]= {10., 12. , 11. , 8. , 10. , 10. , 12. , 11., 8. , 10. };
  Float_t z[N]= {0.9, 0.2, 0.4, 0.7, 0.6, 0.95, 0.33, 1.0, 0.5, 0.99};

  gStyle->SetPalette(1);

  TGraph2D *g=new TGraph2D(N,x,y,z);
  g->SetMarkerStyle(kFullCircle);
  g->SetMarkerSize(2);
  g->Draw("PCOLZ");
  //g->SetMinimum(0.2);
  g->SetMaximum(1.0);
  gPad->SetTheta(0.); gPad->SetPhi(0.);
  g->GetXaxis()->SetTitle("x axis");
  g->GetYaxis()->SetTitle("y axis");
  g->GetZaxis()->SetTitle("z axis");

  c1->Print("Test_Graph_2D.png");
  }

But:

• it could be a coincidence in this case, which maybe (I don’t know) could be replicated in other cases by carefully controlling the range in z and the number of contours;
• the minimum goes to 0, which you might not want in some cases;
• I’m not sure whether the values laying exactly on the value between colours should belong to the lower or higher colour (in the plot, they go with the lower);

so maybe using a lot of contours is better in general.

[couet]

{
   const Int_t N = 10;

   Float_t x[N]= {1. ,  2. ,  3. , 4. ,  5. ,  6.  , 7.   ,  8.,  9. , 10.  };
   Float_t y[N]= {10., 12. , 11. , 8. , 10. , 10.  , 12.  , 11.,  8. , 10.  };
   Float_t z[N]= {0.9,  0.2,  0.4, 0.7,  0.6,  0.95,  0.35,  1.0, 0.5,  0.97};

   gStyle->SetPalette(1);

   TGraph2D *g=new TGraph2D(N,x,y,z);
   g->SetMarkerStyle(kFullCircle);
   g->SetMarkerSize(2);
   g->Draw("PCOLZ");

   g->SetMaximum(1.0);
   g->SetMinimum(0.0);
   g->GetZaxis()->SetNdivisions(-20);
   g->GetZaxis()->SetTitleOffset(1.5);
   gPad->SetTheta(0.01); gPad->SetPhi(0.01);
   g->GetXaxis()->SetTitle("x axis");
   g->GetYaxis()->SetTitle("y axis");
   g->GetZaxis()->SetTitle("z axis");
}