What can we do to make ScottPlot better?

Consider adding nullable annotations as they will increase productivity of using library.
This will clarify what you may expect from function.

For example:

Only those who read source code would know that they may pass null as first parameter

Or:

This may not be obvious that if you pass array with less than 2 elements it will return null

Is this breaking change?

No. Nullable annotation are not breaking existing code. Types remain same, but with some attributes, not changing behavior. It will work with .Net Framework, I've tested it with .Net Framework 4.0 and 4.6 in different projects