interactive-graph: hide click-to-add instructions if points

Summary:
When loading an existing item (in the editor only), the "click to
add points" message would obscure any points that were already present.
Since those points have already been added, this message probably
doesn't need to be on top of them.

Test Plan:
load [this local perseus item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20interactive-graph%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22interactive-graph%201%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B1%2C1%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-10%2C10%5D%2C%5B-10%2C10%5D%5D%2C%22gridStep%22%3A%5B1%2C1%5D%2C%22snapStep%22%3A%5B0.5%2C0.5%5D%2C%22graph%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%2C%22coords%22%3A%5B%5B-3.5%2C-2%5D%2C%5B4%2C-3.5%5D%2C%5B-0.5%2C4%5D%5D%7D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D)
see that the click to add points message appears over the renderer, but
not over the editor

Reviewers: charlie

Reviewed By: charlie

Differential Revision: http://phabricator.khanacademy.org/D10007