Use !== to compare undefined and null

!= returns false rather than true.

Not sure where the best place to put a failing test for this is.
Use yProperty value instead of d.y to fetch y data
Extract defined function to be reused in line and area
Add missing tests to graph
Improve tests for and remove unused method from stack component
Add missing test to stack component
Merge master into fix-display-of-null-data
Merge pull request #206 from alphagov/fix-display-of-null-data

Use !== to compare undefined and null
Upgrade slimmer

Things were breaking on 3.19.0. Probably a 'static' change. Upgrading to
3.20.0 fixed it.
Merge pull request #209 from alphagov/upgrade-slimmer

Upgrade slimmer