how to add additional (energy) abscissa to a TOF spectrum?

Dear rooters,

How can I add additional abscissa (energy scale) to a time-of-flight spectrum?
For 10-m neutron TOF I wrote the attached by using TGaxis, but it's apparently failing...

Thanks in advance,
Kazuyoshi

--
Kazuyoshi Furutaka
furutaka_at_jb3.so-net.ne.jp


{
  gStyle->SetOptStat(0);

  TH1F* tof = new TH1F("tof","10-m neutron TOF: E(eV)=(72.3*10(m)/TOF(#mus))^2",
		       90,10.0,100.0);
  tof->GetYaxis()->SetRangeUser(0,10);
  tof->GetXaxis()->SetTitle("neutron TOF (#mus)");
  tof->Draw();

  TF1* fEne = new TF1("fEne","pow(72.3*10.0/x,2)",10.0,100.0);
  TGaxis* energyAxis = new TGaxis(10,10,100,10,"fEne",510,"-");
  energyAxis->SetTitle("energy (eV)");
  energyAxis->Draw();
}