Bent errors bars

ROOT
1

Hello,

Sorry if this has been asked before, it is probably a common problem but I haven’t been able to find it in search.
I am making a fit with 4 parameters and want to make a plot where I have parameter 0 in x and parameter 1 in y.
Since they are correlated, I need something like a TGraphBentErrors, I think (maybe there is a more automatic way of doing this?). The problem is that I haven’t found any documentation and don’t know the meaning of the last 4 parameters. After trial and error I found that

  protao1900[4]->Fit("ghRL","QNI","",-300,200);
  r_r0[0]=ghRL->GetParameter(0);
  l_r0[0]=ghRL->GetParameter(1);
  er_r0[0]=ghRL->GetParError(0);
  el_r0[0]=ghRL->GetParError(1);
  Double_t matrixp[4][4];
  gMinuit->mnemat(&matrixp[0][0],4);
  tiltxl[0]=-matrixp[0][1]/sqrt(matrixp[0][0]*matrixp[1][1]);
  tiltxh[0]=matrixp[0][1]/sqrt(matrixp[0][0]*matrixp[1][1]);

....

 TGraphBentErrors *r_0=new TGraphBentErrors(3,r_r0,l_r0,er_r0,er_r0,el_r0,el_r0,tiltxl,tiltxh,0,0);

gives approximately what I want as far as the angle of the x errors bars with respect to the horizontal is concerned, however I don’t know what to put in the y errors.

Also, could you expand a bit on the TGraphBentErrors page and section in the guide? It just gives two values of 0.05 for the last parameters and shows two error bars making approximately 45º with the other errors. Maybe it is somewhere and it is very obvious, but I couldn’t understand it.

Thank you

2

Have you seen these :

root.cern.ch/root/html/TGraphBentErrors.html
root.cern.ch/root/html/TGraphPainter.html#GP03c

3

Yes, I have seen both pages (they are equal really), but I didn’t really understand it. So, in my example could you tell what to put in the last two parameters given the correlation matrix please?

4

The last 4 arrays are just deltas allowing to shift the errors bars in the X and Y directions to avoid the overlaps with other errors plots. Nothing else. There are just here for visualisation purpose. Just play with the example given on the TGraphPainter page and you will see the effect.

5

I think I got it, I was think it did something else, didn’t really notice the 0.05 in the x axis, my bad.

Thank you