Allow setting frequency units for psd plots - #32211

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TBH I didn't know that we had this function, looks like it's in here mainly for MATLAB compatibility. Normally we don't deal with units so I'd be tempted to deprecate it, but I think it's fine for a compatibility function. There are some references to Hz in the comments of lib/matplotlib/mlab.py which is reached by that scale_by_freq arg, could you please update those as well?

Normally we don't deal with units so I'd be tempted to deprecate it, but I think it's fine for a compatibility function.

We support physical units through the units module, so I'm curious why that wouldn't work/can't be patched in here. Using unit data should put those units in the label b/c the unit's class handles labeling.