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

MAMISHO/flot,k-joseph/flot,cipix2000/engineering-flot,octavianCraciun/engineering-flot,resnick1223/flot,andig/flot,mcanthony/flot,resnick1223/flot,andig/flot,chaveiro/flot,chiu/flot,chaveiro/flot,saike/flot,k-joseph/flot,Sensirion/flot,fashionsun/flot,resnick1223/flot,gdseller/flot,chaveiro/flot,klarkc/flot,flot/flot,saike/flot,cipix2000/flot,kongx73/flot,k-joseph/flot,chiu/flot,mlitwiniuk/flot,ni-kismet/engineering-flot,magora-suchkov/flot,pepperdata/flot,mlitwiniuk/flot,lolifans/flot,chiu/flot,andig/flot,cipix2000/flot,stanislavdavid/flot,petrvacha/flot,mlitwiniuk/flot,stanislavdavid/flot,colinhunt/flot,matong/flot,saike/flot,sedran/flot,stanislavdavid/flot