void overlay()
{
	auto can = new TCanvas("cc", "cc", 400, 50, 1000, 600);
	auto hpx = new TH1F("hpx", "hpx", 100,-5, 5);
	hpx->FillRandom("gaus");
	hpx->Draw();
	can->Update();
	Double_t ymax = can->GetFrame()->GetY2();
//	Double_t ymax = hpx->GetMaximum();
	auto gr = new TGraph(5);
	Double_t x1 = -1;
	Double_t x2 = 1;
	Double_t y1 = 0;
	Double_t y2 = ymax;
	gr->SetPoint(0,x1, y1);
	gr->SetPoint(1,x2, y1);
	gr->SetPoint(2,x2, y2);
	gr->SetPoint(3,x1, y2);
	gr->SetPoint(4,x1, y1);
	gr->Draw("F");
	gr->SetFillColor(kBlue);
	gr->SetFillStyle(3015);
	can->Update();
}