All spectrographs are non linear to at least some degree and have to be individually calibrated in wavelength. Mike’s spectrograph is fundamentally different to mine for example. His uses a prism and the dispersion is due to refraction whereas mine (and Hugh’s) uses a diffraction grating and the dispersion is due to diffraction. As a result the relationship between wavelength and distance along the spectrum are completely different and in fact run in opposite directions ! (with diffraction the short wavelengths are least deflected whereas in refraction the short wavelengths are refracted the most)
Cheers
Robin