Basic primitives such as Tline, TText do not know about the frame created when drawing histograms and graphs.
In your case simply replace Tline by TGraph as show below
Rene
{
TH1D *hist = new TH1D("histName","histTitle",10,0.,10.);
hist->GetYaxis()->SetRangeUser(0.,2.);
//TLine *line = new TLine(0.,1.,10.,1.);
TGraph *line = new TGraph(2);
line->SetPoint(0,0,1);
line->SetPoint(1,10,1);
line->SetLineColor(kRed);
hist->Draw();
line->Draw("l");
}