Prevent panics when data is flat

[xordspar0]

This avoids division by 0 that occurs when the incoming data is all one value. Instead of panicing, Plot should now plot flat data as expected, in a flat line.

[xordspar0]

I fumbled around with this a bit, and I'm not 100% confident I made the right choices about what to do to avoid the division by zero errors. Please request changes or corrections if you see any way this could be better.

[coveralls]

Pull Request Test Coverage Report for Build 34

• 13 of 13 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.1%) to 76.639%

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Totals
Change from base Build 36:	1.1%
Covered Lines:	187
Relevant Lines:	244

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💛 - Coveralls

[guptarohit]

This looks good 👍