The Magnitude dialog allows you to define the stretch amount as an Analytic Function of position along the reference axis. You can also use a Point graph where you can generate an arbitrary function by manually placing points and setting the spline interpolation. To do this, select Point from the Function drop-down list under the graph. The example graph on the lower right shows the way it was done for the example mesh displayed below. It scales a spherical primitive mesh to a disk with a hump around a certain distance. This modifier is ideal to set up a disk with a complex structure.