Hello ROOTers,
I have 7 points and I wrote a program that allows me to graph them, then from Fit Panel I want to graph them. The code I wrote is as follows:
{
auto c4 = new TCanvas("Note","Retta di calibrazione",200,10,1400,600);
double x[] = {338.1-303.5, 340.4-303.5, 342.5-303.5, 345-303.5, 347.4-303.5, 349.8-303.5, 352-303.5};
double y[] = {5.00911e+00, 1.07664e+01, 2.58592e+01, 2.15048e+01, 3.04902e+01, 4.34195e+01, 5.14712e+01};
double ex[] = {0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1};
double ey[] = {4.36461e+00, 5.73364e+00, 3.96376e+00, 5.41579e+00, 8.09999e+00, 3.91632e+00, 3.96578e+00};
auto ge = new TGraphErrors(7, x, y, ex, ey);
ge->SetTitle("Retta di calibrazione");
ge->GetXaxis()->SetTitle("#theta [Gradi]");
ge->GetYaxis()->SetTitle("[1] [Canali]");
ge->Draw("ap");
The points, except one, seem to lie on a straight line:
Note.pdf (13,6 KB)
yet if I do the fit using the formula [4]*x+[5] it fails, as visible:
TFitEditor::DoFit - using function PrevFitTMP 0x55cd040612d0
FCN=4.32032 FROM HESSE STATUS=FAILED 11 CALLS 357 TOTAL
EDM=3.74884e-12 STRATEGY= 1 ERROR MATRIX UNCERTAINTY 100.0 per cent
EXT PARAMETER APPROXIMATE STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p4 3.20664e+00 4.16915e-02 7.28745e-04 -3.38385e-05
2 p5 -1.05021e+02 1.75982e+00 -2.90538e-02 -6.22644e-07
Why is this happening? I have used the program countless times and it has always done its job well.