Fix for issue 4561

[rosco54]

For backward compatibility after the yaxis.seriesName as-an-array feature was implemented, needed to compute a yLowestValue and yHighestValue for each configured yaxis, not just the effective (visible?) yaxes.

Fixes #4561

Type of change

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• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes