Problem (C++) : 3D surface appears flat on Z. How to enlarge the space between each point of an axe ?

[the-owner]

I want to plot a 3D surface with QT as in this tutorial here.
I have plotted on the Z axe between 0.40 and 0.60 with 20 points (The Z axe is like that 0.40, 0.41, 0.42,..., 0.59, 0.60).
The problem is that there is no enough space between each Z points then the surface appears flat (like a 2D graph).

Is there a function (in QValue3DAxis Class for example) for spreading/enlarging the Z axe ? (by a scale coefficient for example).
(Same question with other words) How to enlarge the space between each point of an axe ?

There is an image of the problem here