Resolve "`compute_only_required_dag_tags` option not working"

This MR fixes a bug with the compute_only_required_dag_tags feature of the ExternalPlotCreator and reworks the corresponding tests.

Can this MR be accepted?

• Fixed the bug No bug found
• Tests added or adjusted
• Pipeline passes
• Changelog entry added
• Approved by @jeremiastraub

Related issues

Closes #105 (closed)