Add full support for custom length units (e.g., microns instead of milimeters)

I loaded in a nrrd file that was properly scaled and it is showing that the scale of the volume is correct, however when I add a scale bar it is in the wrong units. is there a way to change the units on the scale bar? In the screenshot you can see that the volume scale is in microns but the 3D scale bar is in cm.

Thanks for reporting. This is a known issue - see View ruler (horizontal ruler displayed in slice and 3D view nodes) in:

These fixes would probably require contribution from the community - either contribution of development efforts or funding. This issue may also get more attention if it turns out to be a highly requested feature - so I move it now to the “Feature request” category so that people can vote on it.