Allow specification of line interpolation method.
Remove baselines before drawing.

If there's a baseline on a graph that can be updated, we should
remove the baselines before drawing, otherwise we end up with
multiple baselines being drawn.
Merge pull request #95 from mreid-moz/redraw_baselines

Remove baselines before drawing.
Merge pull request #94 from mreid-moz/custom_interpolation

Allow specification of line interpolation method.