_ROOT Version: 6.28.06

_Platform: Windows

*Compiler:* Not Provided

Hello,

I’m trying to force an 2D Fit function though two points by using different error weights. It is not working:

see this simple example:

```
float flat(const float x, const float y, const float mx, const float my, const float z) {
return z + mx*x + my*y;
}
Double_t flat(Double_t *val, Double_t *par) {
Float_t x = val[0];
Float_t y = val[1];
Float_t mx = par[0];
Float_t my = par[1];
Float_t z0 = par[2];
Double_t f = flat(x,y,mx,my,z0);
return f;
}
int simple() {
TGraph2DErrors *gr = new TGraph2DErrors();
gr->SetPoint(0,0,0,50);
gr->SetPointError(0,1e-64,1e-64,1e-64); //SMALL ERROR
gr->SetPoint(1,0,10,52);
gr->SetPointError(1,1e-64,1e-64,1); //BIG ERROR
gr->SetPoint(2,10,0,53);
gr->SetPointError(2,1e-64,1e-64,1); //BIG ERROR
gr->SetPoint(3,10,10,55);
gr->SetPointError(3,1e-64,0.1e-64,1e-64); //SMALL ERROR
TF2 *test1 = new TF2("test1",flat,0,10,0,10,3);
test1->SetParNames("mx","my","z0");
TCanvas *c1 = new TCanvas("c1","c1",1200,800);
gr->Draw("p0");
gr->Fit("test1");
test1->Draw("surf1 same");
std::cout << "SHOULD BE SMALL: " << 50 - test1->Eval(0,0) << std::endl;
std::cout << "SHOULD BE SMALL: " << 55 - test1->Eval(10,10) << std::endl;
std::cout << "SHOULD BE BIGGER: " << 52 - test1->Eval(0,10) << std::endl;
std::cout << "SHOULD BE BIGGER: " << 53 - test1->Eval(10,0) << std::endl;
return 0;
}
```

output is:

```
Processing .\simple.C...
Warning in <Fit>: Abnormal termination of minimization.
FCN=8.92772e+129 FROM MIGRAD STATUS=FAILED 111 CALLS 112 TOTAL
EDM=9.64161e+127 STRATEGY= 1 ERR MATRIX APPROXIMATE
EXT PARAMETER APPROXIMATE STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 mx 4.05000e+00 2.69095e-70 -0.00000e+00 1.13493e+129
2 my 1.70372e+00 1.85692e-70 -0.00000e+00 1.05757e+134
3 z0 4.62743e-09 9.76288e-74 0.00000e+00 -3.55675e+128
SHOULD BE SMALL: 50
SHOULD BE SMALL: -2.53718
SHOULD BE BIGGER: 34.9628
SHOULD BE BIGGER: 12.5
```

Thanks for your hints

Georg