a short question

Hi roottalk;

I've a 3Dplot:

TGraph2D* foo = new TGraph2D("foo.dat"); foo->Draw("surf1");
gPad->SetLogz(1);

I'm interested only at the "floating valley"; I don't want the two wall at the sides.
I understand why they appear, but I don't know how make a plot without them...

Thanks to anyone can help me
Claudio