Allow x2axis/y2axis with no min/max to auto-scale.

Using x2axis/y2axis to add a second axis is deprecated, but we
currently still support it.  Historically we’ve just extended the first
axis’s options with the second’s, but that is a problem when the first
has min/max and the second doesn’t; it will inherit an inappropriate
min/max.

We might want to consider not extending at all, but since that’s a
bigger change we’ll for now just ensure that if no min/max is
specified, we respect that.

Fixes #1228.