Add a meta to hold current process of plot merge

[TheMeinerLP]

Overview

Fixes #3056

Description

Fixes the current duping problem when plots are merged in survival.

Submitter Checklist

• [X] Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• [X] Ensure that the pull request title represents the desired changelog entry.
• [X] New public fields and methods are annotated with @since TODO.
• [X] I read and followed the contribution guidelines.

[TheMeinerLP]

This PR needs be tested. @OneLiteFeather and me have tested locally and sometimes was working and sometimes not. I am now out for today. I still have some improvements on my pc, but I am not sure if there are working. The cause why I am not pushing it, is the locally testing are so buggy. Sometimes the breakpoints are not holding in the right method/line.