Make YAxis seriesName an Array

[j2ghz]

#4247 now allows or requires seriesName to be an array, and that wasn't reflected in the types. I wasn't able to find out which case it is, and even the documentation doesn't mention seriesName being an array, in one place it's a string, in another it's undefined.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes