Contourf workaround for Cartopy 0.19

[rcomer]

🚀 Pull Request

Description

A change at Cartopy 0.19 causes axes extent to be updated when contours are added (see SciTools/cartopy#1780). This is a problem for our special handling for antialiasing in iris.plot.contourf because maps are cropped around the extra contours (see discussion at #4125). @greglucas has written a fix at SciTools/cartopy#1784 but advised that it is unlikely to make it into a release soon. So if we want Iris v3.0.2 to play nicely with Cartopy 0.19, we will probably need a workaround. I propose this one, which just reinstates whatever the axes extent was before the call to contour.

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[greglucas]

Another idea (untested) would be to call
ax.dataLim.update_from_data_xy(extent, ignore=False)
before the contour and then call it with True and the current extents right after the contour call reset it (just swap what None is defaulting too within Cartopy). I think this would get you the expanding part too in case you want that.

[rcomer]

Thanks @greglucas. My version has definitely broken at least one example, so I will give your suggestion a try!

[rcomer]

It may have been more sensible to target master. Now doing that over at #4128.

[rcomer]

This is now in master via #4128 and #4125.

[rcomer]

Now also in v.3.0.x branch via #4130 👍