Offset text should be computed based on lowest and highest ticks, not actual axes limits

[anntzer]

Now that the default margins are nonzero, picking the offset text based on the axes limits rather than the tick limits can have some unesthetic consequences:

plot([1e5, 1e5+1], [0, 1])

Old style (margins=0)

New style (margins>0)

This is not a new issue (it can be reproduced with 1.5.0 by explicitly calling margins), it just got much more salient.