1) made combine plots more permissive
2) avoid 0/0 in divideplots
3) make observable useful
4) mpl_axes_aligner whines if plot's not
   originally centred around 1
5) sharey=True links the y axis together

This is the first in a series of commits to
implement #17

This reverts commit 9c017c2d12cbc3ee1edfb4e0c1d051882db1bbf6.

Thanks to M. Caubet for pointing this out.