GetUymax has problems with log scale

ROOT
1

Dear ROOTers,

I have this annoying problem with the Pad axis limits command, GetUymax(). When I want to plot a vertical line on a plot, I normally do like this:

histo->Draw();
c1->Update();
TLine *l = new TLine(0.5,c1->GetUymin(),0.5,c1->GetUymax());
l->Draw();

This works very well for plots with a linear y scale. However, when I want to use a log scale, I observed the following:

  • If I add the following line before the code above, I get a tiny line, for which Uymax is too small (Uymin looks ok):
c1->SetLogy();
  • If I draw the plot from a macro in linear scale, and then request for SetLogy using the right-click menu of the Canvas, I get a longer line, but still missing something at the top.

Any idea why that happens and maybe how to go around it?

Thanks in advance,

Andrée

2

I’ ll check an let you know.

3

Y max differs in log a linear scales (to show full decades. See plots). The best way is to use NDC coordinates:

{
   TCanvas c;
   hpx->Draw();
   gPad->Update();
   TLine l;   
   Double_t x    = 2.; 
   Double_t xndc = 0.8*((x-c.GetUxmin())/(c.GetUxmax()-c.GetUxmin()))+0.1;
   l.DrawLineNDC(xndc,0.1,xndc,0.9);
   c.SetLogy();
}
4

Hi,

This solution assumes that the histogram frame itself is located between 0.1 and 0.9 NDC in y, which is not always the case (in particular for ATLAS plot style). I can surely put this by hand for the moment, but I think it would be nice to have some way, maybe even a different method if needed, to return the Y max of a log plot, i.e. the maximum visible on the Canvas, which might be different than the linear y max to account for decades etc.

Cheers,

Andrée

5

Sorry, I was lazy. I thought it was easy to make it general. Here is the general case:

void linendc()
{
   TCanvas* c = new TCanvas("c", "c", 800, 800);
   hpx->Draw();
   gPad->Update();
   DrawVerticalLine(2.);
   c->SetLogy();
}

void DrawVerticalLine(Double_t x)
{
   TLine l;
   Double_t lm = gPad->GetLeftMargin();
   Double_t rm = 1.-gPad->GetRightMargin();
   Double_t tm = 1.-gPad->GetTopMargin();
   Double_t bm = gPad->GetBottomMargin();
   Double_t xndc = (rm-lm)*((x-gPad->GetUxmin())/(gPad->GetUxmax()-gPad->GetUxmin()))+lm;
   l.DrawLineNDC(xndc,bm,xndc,tm);
}
6

Hi!
I’m riviving this talk, because I have this doubt.
What happens if instead of a Tline I’m drawing a TGaxis? something like this:

TGaxis *Yaxis = new TGaxis(gPad->GetUxmax(),gPad->GetUymin(), gPad->GetUxmax(), gPad->GetUymax(),gPad->GetUymin(), gPad->GetUymax()/SCALEY ,510,"+L");

I can plot the axis using NDC coordinates, however I have to know the actual values of gPad->GetUymin()" and “gPad->GetUymax()” for the label, because the idea is to compare two histograms, but due to the Pad has Log scale, I cant get these actual values.

Right now I think that a possible solution is SetMaximum(h1->GetMaximum()+ topmargin) and a SetMinimum(h1->GetMinimum() - buttommargin) for the main histogram, but it does not look very practical.

Regards
Andres Navarro.

7

Yes the labels is under your control. It can be anything.

8

Hi, I’m sorry to bump this topic but it’s just about a detail regarding the method described here. It works, but I also need to do c->SetBottomMargin(0.15), and after that gPad->GetBottomMargin() doesn’t get the correct position of the new bottom margin even when doing gPad->Modified(), gPad->Update(), c->Modified() and c->Update(). I would appreciate any help.

Cheers!

9

Can you post a code sample showing the problem ?

10

Trying to do that I realized that the issue was another, and it made things wrong only in combination with SetBottomMargin. Sorry for the noise, the original method works.