add more functionality to the custom line feature

[TatianaBurek]

Describe the Enhancement

In the PR request John recommended to extend the custom lines functionality and add:

• Vertical lines for values that do NOT coincide with one of the independent variable values are not plotted (e.g. vertical line at 12 is plotted, but 13 is not).
• Adding vertical and horizontal lines is good. Would also be nice to add the one-to-one diagonal or perhaps other lines.
• I understand the color chooser selects lines from a rainbow. If possible, I'd prefer the reference lines be added with a default constant color (like gray) instead of the rainbow. But this is definitely a preference and not a requirement.

Need to evaluate these requests and add them if possible.
METplotpy issue: dtcenter/METplotpy#198

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