void grapherrorfit()
{




TFile *_file1 = new TFile("~/Desktop/grapherror.root","READ");

TGraphErrors *g;

g = (TGraphErrors*)_file1->Get("g1_MMGA09L1");




auto *expfunc = new TF1("Exponential fit","[0]+[1]*exp(-x*[2])",0,500); 

expfunc->SetParameters(200,7e03,0.01);

g->Fit(expfunc,"RMS+");

g->Draw("AP");

expfunc->Draw("SAME");



auto fitResult = g->Fit(expfunc,"S");

   auto covMatrix = fitResult->GetCovarianceMatrix();
   std::cout << "Covariance matrix from the fit ";
   covMatrix.Print();

cout << "*******************\n";

std::cout << covMatrix(0,0) << std::endl;
cout << "**********************\n";

double * p = expfunc->GetParameters();
const double * parErrors = expfunc->GetParErrors();


cout << parErrors[0] << " " << parErrors[1]  << " "<< parErrors[2] << endl;
cout << p[0] << " " << p[1] << "   " << p[2] << endl;



double df_dp0 = 1;
double df_dp1 = exp(-p[2] * 100);
double df_dp2 = -p[1]*p[2]*exp(-p[2] * 100);


double sigma_exp = sqrt(    pow(df_dp0,2)*covMatrix(0,0) + pow(df_dp1,2)*covMatrix(1,1)  + pow(df_dp2,2)*covMatrix(2,2)    

                           + 2 * df_dp0 * df_dp1 *covMatrix(0,1) + 2 * df_dp0 * df_dp2 *covMatrix(0,2) + 2 * df_dp1 * df_dp2 *covMatrix(0,2)

                       );


cout << "Real Hit rate error at r=100cm: " << sigma_exp << endl;

cout << "Real Hit rate at r=100cm: " <<  p[0]+p[1]*exp(-p[2]*100) << " +- " << sigma_exp  << endl;


_file1->Close();




}