Errors creating plots/averaging?

Z and y1 are identical in your code sample. So the blue line and black line are identical. Therefore the blue line is hidden under the black one since the black one was created last.

With Tobold's fix to NaN-ing out some z values, I suspect you see a blue/black/blue line.

You don't set anything to NaN in z. I guess what you want to do is.

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