[ROOT] x error bars and fitting (2)

From: Nicolas de Sereville (deserevi@csnsm.in2p3.fr)
Date: Fri Sep 13 2002 - 15:34:22 MEST 

Hi Rooters

When fitting TGraphErrors with a predefined function (e.g. "pol1") the
range fitted seems to be by default [first point; last point of
TGraphErrors].In the case of x error bars, it doesn't seem to take into
account the left part of the error bar for the first point and the right
part of the error bar for the last point.
If the range of the "pol1" function is extended manually to include all
the x error bars, then the fit is improved.

Is it an intented behaviour? if not, would it be possible to change the
default range in order to include all the x error bars with something like :
	low range = first point - 2*xerrorbar
	up range  = last point + 2*xerrorbar

Here is a small script which intends to reproduce this feature.

Thanks in advance

Nicolas

P.S : root 3.02/03 on RH7.1


void test()
{
   gStyle->SetOptFit(1);

   Double_t x[3]  = {1, 2, 3};
   Double_t ex[3] = {0.1, 0.1, 0.1};
   Double_t y[3]  = {1, 2.1, 2.9};
   Double_t ey[3] = {0.1, 0.1, 0.1};

   TGraphErrors *gre = new TGraphErrors(3, x, y, ex, ey);
   gre->Draw("A*");

   gre->Fit("pol1");

   TF1 *f1 = new TF1("f1", "pol1", 0, 4);
//   gre->Fit("f1", "RV+");
}


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