Array parameters for function used in Draw

Hello!

Let's I need to use in Draw function a function with parameter. Just as example:

Double_t partest(Double_t energy) {

   return energy;
}

I can use:
chain->Draw("partest(energy)");

And it works.

Now I have a vector parameter in tree, which normally is used:

chain->Draw("energy[0]")

How to use it in function? If I write

Double_t partest(Double_t* energy) {

   return energy[0];
}

chain->Draw("partest(energy)")

This doesn't work.

Thanks in advance,

-- 
Dmitry A. Shtol                        (SND group)    
Budker Institute of Nuclear Physics