In the early days, Matplotlib was modeled after MATLAB, which uses the major/minor pattern.

I agree that this is an unnecessary limitation. There is a rough idea floating for rewriting the Axis/Tick/Grid logic, which then also would allow arbitrary levels. But that'd be a major endeavor and likely not happening and time soon.

"major endeavor" :D

Thanks for the feedback. Historic decisions ...

What would the API for this even be? Ive never seen a plot with three levels of grids or tick marks, so this seems very specialized. If I wanted something like that, I would just do it by hand with axhline and axvline calls.

see my example. In date scenarios this makes definetly sense to show multiple granularieties to allow for better coloring or so on. Of course i could program that on my one using axhline and so on, but why do i have a library when i program everything myself. I am not a C programmer.