Issues with contour plot of rotated pole data on native grid

Grenville provided me with a datafile, which I can't upload here, but it's small. I'll send it to you via slack.

The surface altitude should be easily plotted using the standard methods, but it creates garbage.

Either the data is not reporting the coordinates correctly, or cf-plot can't quite cope with this situation.

Platform: macOS-14.7.3-arm64-arm-64bit 
HDF5 library: 1.14.4 
netcdf library: 4.9.2 
udunits2 library: /Users/bnl28/mambaforge/envs/madpy/bin/../lib/libudunits2.dylib 
esmpy/ESMF: not available 
Python: 3.11.11 /Users/bnl28/mambaforge/envs/madpy/bin/python3.11
dask: 2024.8.0 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/dask/__init__.py
netCDF4: 1.7.2 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/netCDF4/__init__.py
psutil: 6.1.1 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/psutil/__init__.py
packaging: 24.2 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/packaging/__init__.py
numpy: 1.26.2 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/numpy/__init__.py
scipy: 1.15.1 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/scipy/__init__.py
matplotlib: 3.10.0 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/matplotlib/__init__.py
cftime: 1.6.4 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/cftime/__init__.py
cfunits: 3.3.7 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/cfunits/__init__.py
cfplot: 3.3.0 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/cfplot/__init__.py
cfdm: 1.11.1.0 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/cfdm/__init__.py
cf: 3.16.2 /Users/bnl28/mambaforge/envs/madpy/lib/python3.11/site-packages/cf/__init__.py

MWE:

import cf
import cfplot as cfp
f = cf.read('nzlam.orog')
ff = f[5]
cfp.mapset(proj='rotated')
cfp.con(ff)

with or without the mapset it's garbage.

Obtain the small datafile via Slack, then reproduce the MWE using cf.read('nzlam.orog'), selecting f[5] and calling cfp.con() with and without cfp.mapset(proj='rotated'). Compare the native-grid coordinates and contour output to determine whether the data coordinates or cf-plot handling are responsible; done means the surface altitude plots correctly.