bugfix for making orbit plots when we have a pl rv

[sblunt]

No description provided.

[semaphoreP]

Can you describe the bug and fix? I'm not sure what to be looking for.

[coveralls]

Pull Request Test Coverage Report for Build 5427784093

• 29 of 37 (78.38%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.05%) to 86.37%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
orbitize/plot.py	29	37	78.38%

Files with Coverage Reduction	New Missed Lines	%
orbitize/plot.py	2	80.97%

Totals
Change from base Build 5425350030:	-0.05%
Covered Lines:	5152
Relevant Lines:	5965

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💛 - Coveralls

[sblunt]

@semaphoreP the plotting code was failing when there is a secondary RV measurement in the data file. This fixes that failure.