Dim line terminators when they're not hovered

Currently line terminators are slightly brighter than the rest of
the line that's deselected. Fix that.
Test for null when rendering a stack baseline

Returning null in the function we pass into renderLine allows us to
set the d3 .defined() property of the line conditionally, which lets
the baseline handle null data.
Merge pull request #643 from alphagov/refactor/the-matrix-bugfix-stacked-graphs

Stacked graph bugfixes from matrix refactor