On a cartesian chart why do the lines sometimes disappear?

[ScottMatParker]

The following charts are running the same code off the same .csv file, just using different segments of the data (POC numbers).


POC86 has 496 X,Y Observable points and lines are drawn between points (best seen in the purple data series).
POC87 has 1499 X,Y Observable points and connecting lines are not drawn.
Why? Is it a scale issue? Is there a way to force a line stroke of one pixel? (I've looked and can't find it.)
Am I missing something? It's repeatable, I can flop back and forth with the combobox and 86 gets lines and 87 doesn't.

[ScottMatParker]

I found the problem. Evidently some Y values were coming in NaN, and that stopped the lines.