Fix for issue 4560

[rosco54]

Fix getMinYMaxY() handling of multiple Y values per X value that was broken sometime after revision 3.45.2 following changes to handle chart y axis autoscaling directly within this function.

Fixes #4560

Type of change

• 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