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.