Well, just try the macro from the last post in [url]Vertical shaded area 
You can try to play with it … this way … {
TF2 *f = new TF2("f", "x*x+y*y", -3, 3, -5, 5);
f->SetNpx(100);
f->SetNpy(100);
f->SetLineWidth(1.0);
// ... … and … that way … {
TF2 *f = new TF2("f", "x*x+y*y", -3, 3, -5, 5);
f->SetNpx(10);
f->SetNpy(10);
f->SetLineWidth(1.0);
// ... 