Need to add a horizontal line at y=1

ROOT
2

TLine *line = new TLine(100, 1, 800, 1); or gPad->Modified(); gPad->Update(); // make sure gPad is updated TLine *line = new TLine(gPad->GetUxmin(), 1, gPad->GetUxmax(), 1); or something like TLine *line = new TLine(0.1, 0.5, 0.9, 0.5); line->SetNDC(kTRUE);