Distortion and weird artefacts using polar stereographic projection

[sadielbartholomew]

A user has notified us that contour plots using the south polar stereographic projection (proj="spstere") are distorted in that they are not showing the data correctly, for example in shape and with weird line artefacts appearing in the data displayed across the plots, too - see the example below. They have evidenced that using Python 2 and our final Python 2 version of cf-plot, 2.4.10, everything looks as it should, so something at least has broken either during the migration 2->3 or since then, in this respect.

Though I have only observed this for the south polar case, given the symmetry in the code between these, I suspect the north polar case will suffer from these issues also, but I am yet to test this.

Example

Minimal reproducer

f = cf.read("<filename in question>.nc")
g = f[0][:,0,:,:].squeeze()

cfp.con(g, lines=False)  # see A output

cfp.mapset(proj='spstere', boundinglat=-60)
cfp.con(g, lines=False)  # see B output

cfp.mapset(proj='spstere')
cfp.con(g, lines=False)  # see C output

A, looks as it should

B, some flecks on the data

C, flecks as above plus line artefacts

User images

Some further plots with code similar to the above showing the distortion in shape (note these aren't directly comparable i.e. showing the exact same thing, since the Python 2 case is - going from the title of the plot - from a seasonal mean of the same data):

Working case (Python 2, cf-plot 2.4.10 and cf-python 2.3.2)

Broken case (Python 3, cf-plot 3.3.0 and cf-python 3.13.1)