Don't add padding when there's no last tick.

Flot 0.8 added logic to account for the size of axis tick labels and add
padding around the edges of the plot, to prevent long labels from
sticking out.  But it padded both sides equally, which is incorrect if
the right/top side has no last axis label.

Fixed by allocating padding per-side, and checking whether the last
label would be shown before padding the top or right.  Fixes #1048.
Merge pull request #1200 from dnschnur/xaxis-plot-size-fix

Don't add padding when there's no last tick.