int main()
{
	Double_t x = 0.0, y = 0.0, z=0.0, sigx = 0.5, sigy = 0.5, sigz = 0.5;

	TFile f1("out.root","recreate");
	TCanvas * can1 = new TCanvas();
	can1->SetName("can1");

	TTree * t1 = new TTree("t1","tree");
	t1->Branch("x",&x,"x/D");
	t1->Branch("sigx",&sigx,"sigx/D");
	t1->Branch("y",&y,"y/D");
	t1->Branch("sigy",&sigy,"sigy/D");
	t1->Branch("z",&z,"z/D");
	t1->Branch("sigz",&sigz,"sigz/D");

	t1->Fill();

	x = 1.0;
	y = 1.0;
	z = 2.0;
	t1->Fill();

	x = 2.0;
	y = 4.0;
	z = 8.0;
	t1->Fill();

	x = 3.0;
	y = 9.0;
	z = 18.0;
	t1->Fill();

	t1->Draw("x:z:sigx:sigz","","goff");
	TGraphErrors *gr1 = new TGraphErrors(t1->GetSelectedRows(),t1->GetV1(),t1->GetV2(),t1->GetV3(),t1->GetV4());
	gr1->SetMarkerColor(kRed);
	gr1->SetName("gr1");
	gr1->Draw("AP");

	t1->Draw("x:y:sigx:sigy","","goff");
	TGraphErrors *gr2 = new TGraphErrors(t1->GetSelectedRows(),t1->GetV1(),t1->GetV2(),t1->GetV3(),t1->GetV4());
	gr2->SetMarkerColor(kBlue);
	gr2->SetName("gr2");
	gr2->Draw("P");

	gr1->Write();
	gr2->Write();
	can1->Write();
}