Rob,
A spot diagram is the result of ray tracing a bunch of rays representing a point source to see where they fall on the focal plane. As you suggest, this is often done in a few discrete wavelengths. So long as they all fall within one pixel I'm happy
Having read the book I'm now a big believer in ray-fan plots. They make it pretty easy to "read" what aberrations are present in a scope design. Unfortunately, I've never seen a scope manufacturer supply one...
Cheers,
Rick.