Invisible pcolormesh

[kadrlica]

There is an interesting feature of matplotlib that if the pixels of your pcolormesh are too small compared to the size of your axis they will disappear. This is an issue when plotting small partial healpix maps on all-sky axes (since we dynamically determine the extent of the pcolormesh). This seems like a feature more than a bug, but we could do something to mitigate its impact.