To set the major tick mark interval for the Z-axis:

chSet 'Ztick' 5

You can also set the tick mark positions exactly with:

chSet 'Ztick' VEC

A common use of this would be to generate a logarithmic Z-axis, which is not directly supported by RainPro. Note that if you use this facility, you will completely over-ride any previous settings on the axis range – the axis will be forced to span the gap from the first to the last tick mark you give.