Added new innerElBorderOffset property to base graph class.

This allows us to adjust the position of the graph based on the border width at the bottom.

Removed some crazy code.

Removed some dead code.

Updated tests.
Added test for innerElBorderOffset property.
Merge pull request #704 from alphagov/bugfix/bottom-margin-on-line-graphs

Added new innerElBorderOffset property to base graph class.