@couet @pcanal Here’s the simplest “reproducer.cxx” (add two functions to a graph and then draw it in the traditional way):
{
// root -b -l -q reproducer.cxx
// root --web=off -l -q reproducer.cxx
TF1 *f1 = new TF1("f1", "pol1", 0., 1.);
f1->SetParameters(0.1, 1.);
TF1 *f2 = new TF1("f2", "pol1", 0., 1.);
f2->SetParameters(-0.1, 1.);
TGraph *g = new TGraph(2);
g->SetPoint(0, 0., 0.); g->SetPoint(1, 1., 1.);
g->GetListOfFunctions()->Add(f1);
g->GetListOfFunctions()->Add(f2);
g->Draw("A*");
}