I)

TF1 *f1 = new TF1("f1","[0]*sin([1]*x)/ x",0.,10.);
f1->SetParameter(0,1); f1->SetParameter(1,1);
f1->Draw();

TF1 *f2 =  new TF1("f2","[0]*sin(2 * [1] * x)/ (2 * x)",0.,10.);
f2->SetParameter(0,1); f2->SetParameter(1,1);
f2->Draw("SAME");

II)

TGraphErrors *gr = new TGraphErrors("myFile.txt");
gr->SetTitle("Measurements;T;arbitrary units");
gr->Draw("AP")