Fix legend crossing out

[DL6ER]

What does this implement/fix?

See title. Fixed by also checking item.hidden property when deciding to skip legend regeneration.

Example:

We can click on, e.g., A to hide A. Hiding the red chart area is working but the chart's legend is not updated:

This is fixed by this PR (see unticked box + crossed out text):

This was originally broken in 9c1f310 of #2811

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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