I'm a bit confused about the unit mechanics in yt right now. I noticed there's a convert_function for fields, but there are also the units, time_units, and conversion_factors attributes for StaticOutput.
For the Nyx frontend, I have added convert_function's for each field to convert the cosmological units into CGS. This works in my fork now and everything is in cgs and plots fine. Now I know I need to update the _set_units method of NyxStaticOutput, but I'm not sure what to do with those attributes and how they are used later.