Sum of TGraph integrals

[Wile_E_Coyote]

Numerical integration with ROOT. (Function defined in points)

Be aware that the TGraph::Integral name is extremely misleading. It calculates the “pseudo-integral” of a closed polygon, NOT the “integral” of the function. Try this: { TGraph *g = new TGraph(); g->SetPoint(0, 1.0, 1.0); g->SetPoint(1, 2.0, 0.0); g->SetPoint(2, 3.0, 2.0); g->SetPoint(3, 4.0, 1.0); g->Draw("AL*"); std::cout << "ROOT's pseudo-integral = " << g->Integral(0, 3) << std::endl; }

Plotting the differences between two graphs

Note that the solution proposed by Olivier assumes that both graphs have exactly the same numbers of points and that their X coordinates are exactly the same (for all points). If any of the above conditions is not met, try: { // Note: this macro needs ROOT 6 // define some variables used below Double_t x[100], y[100]; Int_t n; // create the first graph n = 22; for (Int_t i = 0; i < n; i++) { x[i] = i * TMath::Pi() / (n - 1.); y[i] = TMath::Sin(x[i]); } TGraph *g…