This demonstration explains how to display axis with a VolumeData in a VolumeRender node. Various kind of axis are available such as single axis, XYZ-axis, Cube-Axis from MeshViz.
A combo box provides the ability to switch between the available axis types:
SoVolumeData, SoVolumeRender, PoLinearAxis, PoGroup3Axis3, PoAutoCubeAxis