WCSAxes fails to draw grid along separable coordinate after slicing

[Cadair]

Running the following code:

from astropy.io import fits
from astropy.wcs import WCS
import matplotlib.pyplot as plt
import numpy as np



header = """
WCSAXES =                    3 / Number of coordinate axes
CRPIX1  =                  1.0 / Pixel coordinate of reference point
CRPIX2  =                386.5 / Pixel coordinate of reference point
CRPIX3  =                 32.0 / Pixel coordinate of reference point
PC2_2   =       0.999988496304 / Coordinate transformation matrix element
PC2_3   =       -0.13517896595 / Coordinate transformation matrix element
PC3_2   =    0.000939457726278 / Coordinate transformation matrix element
PC3_3   =       0.999988496304 / Coordinate transformation matrix element
CDELT1  =    1.29800001159E-12 / [m] Coordinate increment at reference point
CDELT2  =  4.6208333333333E-05 / [deg] Coordinate increment at reference point
CDELT3  =  0.00055428900205556 / [deg] Coordinate increment at reference point
CUNIT1  = 'm'                  / Units of coordinate increment and value
CUNIT2  = 'deg'                / Units of coordinate increment and value
CUNIT3  = 'deg'                / Units of coordinate increment and value
CTYPE1  = 'WAVE'               / Vacuum wavelength (linear)
CTYPE2  = 'HPLT-TAN'           / Coordinate type codegnomonic projection
CTYPE3  = 'HPLN-TAN'           / Coordinate type codegnomonic projection
CRVAL1  =    1.33168328015E-07 / [m] Coordinate value at reference point
CRVAL2  =   -0.029883055555556 / [deg] Coordinate value at reference point
CRVAL3  =     0.22718416666667 / [deg] Coordinate value at reference point
LONPOLE =                180.0 / [deg] Native longitude of celestial pole
LATPOLE =   -0.029883055555556 / [deg] Native latitude of celestial pole
"""


wcs = WCS(header=header)
data = np.random.random((64, 772, 2055))


ax = plt.subplot(projection=wcs, slices=('x', 0, 'y'))
ax.imshow(data[:,0,:])
ax.coords[0].grid()

plt.show()

results in this error:

Traceback (most recent call last):
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/backends/backend_qt5.py", line 505, in _draw_idle
    self.draw()
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py", line 388, in draw
    self.figure.draw(self.renderer)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/figure.py", line 1709, in draw
    renderer, self, artists, self.suppressComposite)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/image.py", line 135, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/core.py", line 454, in draw
    super().draw(renderer, inframe=inframe)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/artist.py", line 38, in draw_wrapper
    return draw(artist, renderer, *args, **kwargs)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/axes/_base.py", line 2647, in draw
    mimage._draw_list_compositing_images(renderer, self, artists)
  File "/home/stuart/.virtualenvs/sunpy-dev/lib/python3.7/site-packages/matplotlib/image.py", line 135, in _draw_list_compositing_images
    a.draw(renderer)
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/core.py", line 43, in draw
    self.axes.draw_wcsaxes(renderer)
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/core.py", line 411, in draw_wcsaxes
    coord._draw_grid(renderer)
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/coordinate_helpers.py", line 533, in _draw_grid
    self._update_grid_lines()
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/coordinate_helpers.py", line 868, in _update_grid_lines
    pixel = self.transform.inverted().transform(xy_world)
  File "/home/stuart/Git/astropy/astropy/visualization/wcsaxes/wcsapi.py", line 231, in transform
    raise ValueError(f"Expected {self.wcs.world_n_dim} world coordinates, got {len(world)} ")
ValueError: Expected 3 world coordinates, got 2 

without the grid call it's fine.

cc @astrofrog

[pllim]

@Cadair , you are not using the template that we stole from you. 😝

What are the versions?

[Cadair]

This is master, should have specified.

I am in the habit of ignoring that template already 😉

[dstansby]

I think the coordinate isn't separable? Printing the transformation gives the below, with correlations between pixel dim 0 and world dims {1, 2}.

SlicedLowLevelWCS Transformation

This transformation has 2 pixel and 3 world dimensions

Array shape (Numpy order): None

Pixel Dim  Axis Name  Data size  Bounds
        0  None            None  None
        1  None            None  None

World Dim  Axis Name  Physical Type                   Units
        0  None       em.wl                           m
        1  None       custom:pos.helioprojective.lat  deg
        2  None       custom:pos.helioprojective.lon  deg

Correlation between pixel and world axes:

           Pixel Dim
World Dim    0    1
        0  yes   no
        1   no  yes
        2   no  yes