When graphing via graphie/raphael, truncate our graphs at 500,000.
When graphing via graphie/raphael, truncate our graphs at 500,000.

Summary:
Large x or y coordinates in svg files overflow the firefox svg
renderer, leading to bad output.  We clip such coordinates to 500000.
This is safe since 500K is going to be outside our viewport in any
case.

Fixes https://app.asana.com/0/25429045350268/186328770662859

Test Plan:
Ran `make serve` and visited
   http://localhost:9000/
I then clicked 'add a widget...' -> 'grapher', clicked on the triangle
next to 'grapher1', and clicked on the exponent 'available functions'
(5th option), and unclicked the line function (1st option).  I then
went to the graph and put one green point at (-1, 1) and another at
(0, 5).

Before this fix, on firefox, a bunch of blue lines would show up at
x=7 through x=9.  After this fix, no such lines show up.

Reviewers: emily

Reviewed By: emily

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D31446

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