TTree::Draw

Hello there!

Is it possible to draw a histogram from a tree, where I do some calculations to the variables.
i.e.
TTree->Draw("(Frequency-b)*a[n]:Phase");

where my branches are Frequency and Phase and b and a are coefficients. I have tried it in my code and it says that a and b are bad numerical expressions.

Thanks
Belen

Please do not submit the same question to multiple threads. See my answer at:
root.cern.ch/phpBB2/viewtopic.php?t=1349

Rene