Ra/Dec axes not appearing with WCS

Description

When using an image where WCS coords are aligned with the CCD coords the RA/Dec axes do not appear

Expected behavior

Have Ra/Dec axes when plotting with projection=wcs

Actual behavior

The Ra/Dec axes do not appear

Steps to Reproduce

1.Import relevant packages

import matplotlib.pyplot as plt
import os
from astropy.wcs import WCS
from astropy.io import fits
from astropy import units as u
from astropy.visualization import ZScaleInterval
from astropy.utils import data

1. Download the data

remote_url = 'https://lsst.ncsa.illinois.edu/~scichris/postISRCCD_LSSTComCam_r_03_CC_H_20211231_006002_R22_S01_run1.fits'
file_path = data.download_file(remote_url)
print(f'Using {file_path}')

1. Get the WCS and data from fits file

hdul = fits.open(file_path)
wcs = WCS(hdul[1].header)
data = hdul[1].data
zscale = ZScaleInterval()
vmin, vmax = zscale.get_limits(data)

1. Plot the data

fig = plt.figure(figsize=(10,10))
ax = fig.add_subplot(1, 1, 1, projection=wcs)
ax.imshow(data,  origin='lower', vmin=vmin, vmax=vmax,cmap='Greys',)
ax.grid(color='yellow', ls='solid')
ax.set_xlabel('Right Ascension')
ax.set_ylabel('Declination')

1. Other plot commands tried (that were recommended but did not solve the issue):

# obtain the axes 
ra = ax.coords['ra']
dec = ax.coords['dec']
# change the spacing to be more narrow
ra.set_ticks(spacing=2 * u.arcmin, color='white')
dec.set_ticks(spacing=2 * u.arcmin, color='white')
# set the axis label
ra.set_axislabel('RA', minpad=0.3)
dec.set_axislabel('DEC', minpad=0.3)
# se the tick label 
ra.set_ticklabel_position('b')
ra.set_axislabel_position('b')

System Details

Linux-3.10.0-1160.15.2.el7.x86_64-x86_64-with-glibc2.10 Python 3.8.8 | packaged by conda-forge | (default, Feb 20 2021, 16:22:27) [GCC 9.3.0] Numpy 1.20.2 astropy 4.2.1 Scipy 1.6.3 Matplotlib 3.4.2

[embray]

Thanks for the detailed report. I was able to reproduce the problem. While playing around with it I got it to correctly display the axis labels and titles exactly once and then it never worked again. I have no idea how.

[suberlak]

Ok, thank you! I expected RA to be on the bottom, but if I use

ra =  ax.coords['ra']
ra.set_ticklabel_position('b')
ra.set_axislabel_position('b')
ra.set_axislabel('RA')

I get nothing. The only way for axes to appear was

ra = ax.coords['ra']
ra.set_ticklabel_position('l')
ra.set_axislabel_position('l')
ra.set_axislabel('RA')
dec  = ax.coords['ra']
dec.set_ticklabel_position('b')
dec.set_axislabel_position('b')
dec.set_axislabel('DEC')

It's interesting, reading issue #7180 I see some similarities. It is phosim-simulated data so it is possible that the WCS is off (in fact that's what I was fixing by changing the `ROTANG` in the header), but perhaps I also needed to change the CD matrix. Is it possible to rotate the CD matrix with 'astropy.wcs' (same question as https://github.com/astropy/astropy/issues/7180#issuecomment-520088274 but I didn't find an answer in the thread) . Thanks!