ZigZag should not plot line when no thresholds are met

[DaveSkender]

the [potential] problem

• Confirm that the last line of ZigZag is not plotting extra interim points before thresholds are met.
• Add extra unit tests that confirm unusual scenarios - see eval-zigzag branch.

This is a follow-on to #600

to reproduce

See results from the unit test in the above branch. There are some extra zigs at the end, but it could be my unit test is bad or am not interpreting it correctly.

expected behavior

The last line is straight to the last point if it is not long enough to reach a threshold.

[DaveSkender]

Lines were plotting when no confirmed extreme points were present in the dataset.

[github-actions]

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