fitting unsorted data

Voropaev S.G. (voropaev@inp.nsk.su)
Fri, 29 Aug 1997 12:08:38 +0700

Hello,
I have new problem with fitting unsorted data. I am trying to fit unsorted
array X,Y to line. I use object TGraph. Parameters of line is well, but error
is very strange.
Test macro with only 5 data point. (In real life number of data point is
larger, but problem still exist):

{
float Xarr[5];
float Yarr[5];
Xarr[0] = 0.062893; Yarr[0] = 0.584000;
Xarr[1] = 0.000000; Yarr[1] = 0.646000;
Xarr[2] = 0.062893; Yarr[2] = 0.572000;
Xarr[3] = 0.125786; Yarr[3] = 0.497000;
Xarr[4] = 0.125786; Yarr[4] = 0.533000;
// Xarr[5] = 0.000000; Yarr[5] = 0.626000;
// Xarr[6] = 0.125786; Yarr[6] = 0.529000;
// Xarr[7] = 0.251572; Yarr[7] = 0.432000;
// Xarr[8] = 0.125786; Yarr[8] = 0.534000;
// Xarr[9] = 0.251572; Yarr[9] = 0.441000;
// Xarr[10] = 0.188679; Yarr[10] = 0.480000;
// Xarr[11] = 0.000000; Yarr[11] = 0.627000;
// Xarr[12] = 0.377358; Yarr[12] = 0.338000;
// Xarr[13] = 0.188679; Yarr[13] = 0.467000;
// Xarr[14] = 0.251572; Yarr[14] = 0.405000;
// Xarr[15] = 0.000000; Yarr[15] = 0.646000;
// Xarr[16] = 0.503145; Yarr[16] = 0.252000;
// Xarr[17] = 0.251572; Yarr[17] = 0.366000;

c1 = new TCanvas("c1","proba");

TF1 *f1 = new TF1("f1","pol1");
f1->SetRange(0,0.15);

TGraph *tg = new TGraph(5, Xarr, Yarr);
tg->Fit("f1","R","AP");
tg->Draw("A*");
}

Result of fitting: p0 = 0.64 +- 0.85 !, p1 = -1.04 +- 9.5 !!!

Where is error? It is my error or not?

I use ROOT 1.02/00 1 august 1997 ( Valery Fine version) on Windows NT

With best regards Voropaev S.G.