PR Summary

We set the scale for secondary_axis for each draw in case the parent scale has changed (i.e. from linear to log), but this means that we can't set the locators and formatters for the secondary axis. This change keeps track of the parent scale and only resets the locators and formatters if the parent scale changes.

Closes #14443

import matplotlib.pyplot as plt
import numpy as np
import matplotlib.ticker as mticker

fig, ax = plt.subplots(constrained_layout=True)
x = np.arange(0, 360, 1)
y = np.sin(2 * x * np.pi / 180)
ax.plot(x, y)
ax.set_xlabel('angle [degrees]')
ax.set_ylabel('signal')
ax.set_title('Sine wave')

def deg2rad(x):
    return x * np.pi / 180

def rad2deg(x):
    return x * 180 / np.pi

secax = ax.secondary_xaxis('top', functions=(deg2rad, rad2deg))
secax.set_xlabel('angle [rad]')

ax.minorticks_on()
secax.minorticks_on()

plt.show()

now works as expected...

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way