Set first axis number to draw

[Wile_E_Coyote]

Try:

mg_comp_Stz_Q2->Draw("ALP");
gPad->Modified(); gPad->Update();
mg_comp_Stz_Q2->GetHistogram()->GetXaxis()->SetLimits(0.5, 69.5); /* or maybe (1., 70.) */
gPad->Modified(); gPad->Update();