Add point-plotting to interactive-graph widget
Add point-plotting to interactive-graph widget

Test Plan: Drop perseus.min.js into webapp, create an item using interactive-graph widget with the "point" type, save and preview item. Test no answer, incorrect answer, and right answer.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2604

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