Vertical scales of 2D spectra: Consider DOSY spectra / series of 1D spectra.

In DOSY spectra, the F1 scale is not Hz or ppm, but a diffusion coordinate. In some cases, 2D spectra are series of 1D spectra. The unit in the F1 dimensions may be an experiment number, a temperature, a spacial coordinate in the tube (slice-selective experiment), or fuction of a delay (T1, T2 measurement). The scale should completely "user definable", (values and units). Default using ppm (and optionally) Hz, should be default, but should allow more flexibility.