Distinguish between the first and innermost axis.

The fix for #1056 caused a regression where grid lines were drawn for
the innermost axes on both sides instead of just the first axis.
Fixed by properly distinguishing the first axis in each direction from
the innermost one on each side.  Fixes #1075.
Merge pull request #1198 from dnschnur/axis-grid-fix

Distinguish between the first and innermost axis.