Resolve "Add plot config shortcut for cases like `foo.based_on: foo`"

This MR ...

• adds the my_plot: inherit shortcut syntax during plot config inheritance (#353 (closed))
• allows empty YAML nodes when plotting from file (#274 (closed))

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #353 (closed), #274 (closed)