This tutorial demonstrates how to use secondary axis.
SecondaryTickGenerator utilizes "primaryValues-to-secondaryValues" mapping to generate ticks/labels linearly spaced in secondary model space, so that distance between major ticks/labels in the space is equal to "majorStep" (or "minorStep" for minor ticks/labels correspondingly).
Note, that linear spacing in secondary space does not mean linear spacing in primary space (which is model space set on the axis's parent group).
The example contains one axis (left-most) with NumericLinearTickGenerator as a reference one. The other three axes are built on SecondaryTickGenerator. All of them have the same mapping and major/minor steps. However, the first of them shows ticks and labels for depth ranges growing downwards; the second, for depth ranges growing upwards; and the last, for all of the depth ranges in the "primaryValues-to-secondaryValues" mapping