Plotting projections example

Two commits here
The first one delivers quite a nice picture, while the second one extends it to make two plots in different display projections.
I believe the _plots_ in the second case make the issues clearer, but I'm anticipating that someone may not like the extra layer that it introduces into the code.

The only _technical_ downside of the second commit, I believe, is that it is no longer possible to add gridlines.
If we do decide to revert to a single plot, there are some improvements to comment texts in the "second commit" that should probably be back-ported.