Cubic polynomial fit 4 points has uncertainties

ROOT
3

Hi Lorenzo,

Thank you. I tried to change x points as:

void MyTest()
{
    double x[4] = {328, 329, 330, 331};
    double y[4] = {813.128, 690.941, 592.383, 413.539};
    TGraph *gr = new TGraph(4, x, y);
    gr->Draw("AP*");
    gr->Fit("pol3");
}

And it looks wee now.

c1Test_x300
c1Test_x300698×474 6.06 KB

Then if I change x points by 100 more like:

void MyTest()
{
    double x[4] = {428, 429, 430, 431};
    double y[4] = {813.128, 690.941, 592.383, 413.539};
    TGraph *gr = new TGraph(4, x, y);
    gr->Draw("AP*");
    gr->Fit("pol3");
}

Then it gives me errors:

c1Test_x400
c1Test_x400853×412 11.6 KB

It’s strange. If I use MS-Excel or LibreOffice, I can easily get a correct results:

c1Test_x400_MS_Excel
c1Test_x400_MS_Excel746×450 39.1 KB

So how should I set ROOT to fit “pol3” to 4 points always correctly?

Thank you,
Kailong