Example of plotting coordinates on an all-sky map (final pull request)

This should be the final pull request as discussed in issue #2726. There were some problems with my repo, so I had to re-create my branch and so github doesn't allow me to update the original pull request.
I concur.  All this kind of detail is best handled internally by the wonderful abstractions we've provided people.  It's far too easy to make mistakes doing this kind of manipulation by hand.