How to draw objects on graph (canvas) according to its size?

[p73]

I want to draw a vertical line on my graph. But this:

mgr->GetXaxis()->SetTitle("U_{apd}, V");
  mgr->GetXaxis()->CenterTitle();
  mgr->GetXaxis()->SetTitleOffset(1.4);
  mgr->GetYaxis()->SetTitle("Gain");
  mgr->GetYaxis()->CenterTitle();
  mgr->Draw("AP");                             
//Draw vertical line with text at Ub
TLine* l = new TLine(fitpar[2], 0, fitpar[2], 300);
  l->Draw("same");       
TText* ub = new TText();
  ub->DrawText(fitpar[2]-30., 2, "Ub");
                               
c->SetLogy();
c->SetGrid();    

provides not very beautiful picture (see Fig.) So how do I know the canvas or graph (which it should be) y-range?

[couet]

GetX1(), GetX2(), GetY1(),GetY2() will return you the pad coordinates

[p73]

I tried this but do not able to understand the result (see Fig.).

mgr->GetXaxis()->SetTitle("U_{apd}, V");               
  mgr->GetXaxis()->CenterTitle();                        
  mgr->GetXaxis()->SetTitleOffset(1.4);                  
  mgr->GetYaxis()->SetTitle("Gain");                     
  mgr->GetYaxis()->CenterTitle();                        
  mgr->Draw("AP");                                       
                                                               
c->SetLogy();                                          
c->SetGrid();                                          
                                                               
//Draw vertical line with text at Ub                   
TLine* l = new TLine(fitpar[2], 0, fitpar[2], c->GetY2());         
  l->Draw("same");                                       
TText* ub = new TText();                               
  ub->DrawText(fitpar[2]-30., 2, "Ub"); 

How to interpret the return value of GetY2()?

[Wile_E_Coyote]

Try:TLine *l = new TLine(fitpar[2], TMath::Power(10., gPad->GetFrame()->GetY1()), fitpar[2], TMath::Power(10., gPad->GetFrame()->GetY2()));

[couet]

Try this:

{
   TCanvas *c  = new TCanvas("c","c",0,0,500,500);

   const Int_t n = 20;
   Double_t x[n], y[n];
   for (Int_t i=0;i<n;i++) {
      x[i] = 1+i*0.1;
      y[i] = 10*sin(x[i]+0.2);
   }
   gr = new TGraph(n,x,y);
   gr->Draw("ACP");
   gPad->Update();

   double yl1 = c->GetFrame()->GetY1();
   double yl2 = c->GetFrame()->GetY2();
   TLine l(1.,yl1,1.,yl2);
   l.Draw();

   gPad->SetLogy();
}

[p73]

It seems it works only if l is a pointer to TLine but not the TLine instance.

[couet]

@p73

It seems it works only if l is a pointer to TLine but not the TLine instance.

Yes if your macro is named you should use a pointer.

[p73]

OK.
So Do you have an idea why in my initial macro it works different?

[Wile_E_Coyote]

See where you call “c->SetLogy();”.

[p73]

You are right. SetLogy() is the thing. But how to deal with this if I want to save the file in .png format:

gSystem->ProcessEvents();
TImage* img = TImage::Create();                                    
  img->FromPad(c);
  img->WriteImage(figName);
delete img;
delete l;
delete ub; 
c->Clear();

If I try to set log-scale before the above piece of code the line becomes shorter.

[p73]

Sorry, guys. I did not mention that this part of code are in the loop. So I should turn off log-scale after each iteration before the next. How to do this? I tried SetLogy(kFALSE) but it does not seem to work.

[couet]

Can you send a short running reproducer of your problem ?
You only sent pieces of code … that’s hard to debug… Send something with the full logic. Not necessarily the original code (which might be too long) but a small macro rerproducing the effect. Thanks

[p73]

OK. In a few minutes.

[Wile_E_Coyote]

If you are sure that “log Y” is always effective when you create the “TLine” then use the trick from my first post here.

[p73]

I solved this problem. But not sure in the best way. Here is code:

void test(){                                                               
  TGraph* mgr = new TGraph(3);
    mgr->SetMarkerStyle(20);
  
  for (Int_t i = 0; i < 3; i++){
    TCanvas* c = new TCanvas("c", "c", 500, 300);
    mgr->SetPoint(i, i+1, i+2);
    
    mgr->GetXaxis()->SetTitle("U_{apd}, V");
    mgr->GetXaxis()->CenterTitle();
    mgr->GetXaxis()->SetTitleOffset(1.4);
    mgr->GetYaxis()->SetTitle("Gain");
    mgr->GetYaxis()->CenterTitle();
    mgr->Draw("AP");
    
    gPad->Update();
    
    //Draw vertical line with text at Ub
    TLine* l = new TLine(2., c->GetFrame()->GetY1(),
                         2., c->GetFrame()->GetY2());
    l->Draw();
    TText* ub = new TText();
    ub->DrawText(2, 3, "Ub");
    
    gPad->SetLogy();
    gPad->SetGrid();
    
    gSystem->ProcessEvents();
    TImage* img = TImage::Create();
        img->FromPad(gPad);
        img->WriteImage("test.png");
    delete img;
    delete l;
    delete ub; 
    delete c;
  }
}

You see that every iteration I have to create and delete pointer to canvas.

[couet]

Yes it can be simpler:

void test(){
   TGraph* mgr = new TGraph(3);
   mgr->SetMarkerStyle(20);
   TCanvas* c = new TCanvas("c", "c", 500, 300);
   TLine* l = new TLine();
   TText* ub = new TText();

   for (Int_t i = 0; i < 3; i++) {
      mgr->SetPoint(i, i+1, i+2);
      mgr->GetXaxis()->SetTitle("U_{apd}, V");
      mgr->GetXaxis()->CenterTitle();
      mgr->GetXaxis()->SetTitleOffset(1.4);
      mgr->GetYaxis()->SetTitle("Gain");
      mgr->GetYaxis()->CenterTitle();
      mgr->Draw("AP");
      gPad->SetLogy(0);

      gPad->Update();

      //Draw vertical line with text at Ub
      l->DrawLine(2., c->GetFrame()->GetY1(),
                  2., c->GetFrame()->GetY2());
      ub->DrawText(2, 3, "Ub");

      gPad->SetLogy(1);
      gPad->SetGrid();

      c->Print(Form("test_%d.png",i));
   }
}