This is figure 6 that implies at least five previous so there must be much more code than just this snippet. As jonas shows, it doesn't happen in isolation as a rule so one must presume it's a figment of some previous actions left over that have confused the line color property being retrieved automagically.
Can you start a new session and just do the one plot relatively conveniently? I'm guessing that one would be normal and the conclusion would be that to try to fix this in the existing session would be to introduce a fixup for some previous error and so the better route would be to start from a clean session if it isn't two days worth of work but can be rerun quickly.
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