Make plotter hairlines display under other elements on the graph
Make plotter hairlines display under other elements on the graph

Summary:
Updates the plotter hairline styles to match the mockups, where the hairlines are below other elements such as bars and movable points.

https://app.asana.com/0/132409062806753/166319175082274

Test Plan: - Verified in Perseus demo that hairlines now are covered up by other elements on the graph

Reviewers: kevinb

Reviewed By: kevinb

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D29867

ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,learningequality/perseus