I think the option is not implemented for this kind of plot – at least I couldn’t find it. But you can try using SetNdivisions on the z axis; add these lines (before drawing, but also works after):
h1->SetNdivisions(1,"z");
h5b->SetNdivisions(1,"z");
h7b->SetNdivisions(1,"z");
But you lose the tick marks on z. To show them you would have to make and draw your own axis; see here:
https://root.cern.ch/doc/master/classTGaxis.html