PR Summary

Fixes an issue on gitter by @Tillsten whereby the secondary_axes doesn't do ticks properly for non-linear axes.

The issue here is that we really need to use the inverse to position ticks, not the forward function:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0.01, 90, 91)

def xsq(x):
    return((x)**2)

def sqx(x):
    return(np.sqrt(x))

fig, ax  = plt.subplots(1, 1, figsize=(4, 3), constrained_layout=True)
ax.plot(sqx(x), x)
ax.set_ylim([0.001, 90])
secax = ax.secondary_yaxis('right', functions=(sqx, xsq))
plt.draw()
ax.set_xlabel("$x^{1/2}$")
ax.set_ylabel("x")
plt.show()

Before

This is what we would want a sqrt(x) scale to normally look like....

After

... but for a secondary axis we want the values to match up, so we really need to specify the position with the inverse:

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