Click the ‘Custom Stack Profile’ toggle in the ‘Generator’ panel. To edit a profile, click the ‘...’ button. This brings up the Stack Profile Editor window:
Initially, the profile lies on the stack boundary and has no effect. Drag the anchor points to the left to form a profile. The curve is a literal representation of the profile. When you release the mouse, the form is rebuilt.
Alternatively, a growth curve may be applied to the stacks. See Growth Curves for more information.