from itertools import count
from math import copysign

# https://pypi.org/project/prefixed/
from prefixed import Float

from matplotlib.ticker import ScalarFormatter

class FrequencyFormatter(ScalarFormatter):
    def format_data(self, value):
        # Add the Hz suffix
        return '{:H}Hz'.format(Float(value))
    def _set_order_of_magnitude(self):
        super()._set_order_of_magnitude()
        c = abs(self.orderOfMagnitude)
        # Search 0, -3, 3, -6, 6, -9, 9 etc until the best scientifically
        # prefixed oom is found, and then set self.orderOfMagnitude to it
        for sciOom in count(3,3):
            if c < sciOom:
                self.orderOfMagnitude = copysign(sciOom, self.orderOfMagnitude)
                break

# fig,ax...

ax.yaxis.set_major_formatter(FrequencyFormatter(
    # I'm not necessarily interested in this, but without this,
    # ScalarFormatter.get_offset doesn't use my own .format_data function to
    # format also the offset.
    useMathText=True
))