Bug report

Bug summary

I get a warning related to FixedFormatter even though I am not setting a formatter.

Thanks to user Rational-IM on StackOverflow and the good folks at pandas for isolating and reproducing the bug.

Code for reproduction

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.scatter(0.5, 0.5)
ax.set_xticklabels(['0', r'$T/2$', r'$T$'])

# Optional line
ax.xaxis.set_major_locator(FixedLocator([0, 0.5, 1]))

None

Actual outcome

<ipython-input-3-52a3699e4278>:5: UserWarning: FixedFormatter should only be used together with FixedLocator
  ax.set_xticklabels(['0', r'$T/2$', r'$T$'])

Expected outcome

Works on 3.2.2 with no warning.

Matplotlib version

• Operating system: Win 10
• Matplotlib version: 3.3.2
• Matplotlib backend: module://ipykernel.pylab.backend_inline
• Python version: 3.8.5
• Jupyter version (if applicable): 6.1.4
• Other libraries: none

Installed from conda