When does "copying" a math diagram become plagiarism?

Paraphrasing Simon Peyton Jones here, credit is like love. Giving it to more people does not diminish it or you.

Even if you recreated an image yourself, there’s literally no reason to not attribute your inspiration for it (apart from your own ego).

The threshold for plagiarism is vague at times, but if you saw someone’s work, liked it and built upon it, give them credit.

I am not a lawyer. However, this is how it presents itself to me:

I think it is necessary to distinguish the "citation" from the "copyright" aspect:

Citation: A complete recreation of a diagram to convey a (semi-)formal idea is like the reproduction of a formula. I would not assume copyright violation applies here, but it definitely needs to be cited if there is any original contribution in the diagram (e.g. it is the first time that something is visualised in a particular way etc.) rather than a well-known and -used standard diagram.

In Greek mathematics, diagrams often took the role that formulas take today, and this is still often the case today: exact sequences, Feynman diagrams, Organic Chemistry etc. all fall into this category. In short, if your diagram falls into a category equivalent to any of these, recreate and cite.

Copyright: If the diagram itself is not original, but you are copying the style, possibly colour scheme etc., then recreating it may fall under copyright violation and you need permission from the copyright holder.

Plagiarism is the act of taking someone else's work and passing it off as your own.

If this happens explicitly (i.e. you claim originality) then things are pretty clear cut.

The problem comes when you fail to clarify that a piece of work is not your own. Leaving the authorship of an idea ambiguous is where you get into trouble.

Thus, at a bare minimum, you need to give credit. Depending on what you're doing, this might be a formal citation, or a clear notice of the source. In a presentation, this might be an annotation on the slide or a clear verbal indication of where you got the idea.

Copyright takes things further. This moves from academic conduct into legal waters.

The specifics here depend on your country. Copying someone's work with appropriate citations may be covered by "fair use" clauses in your local copyright laws which permit copying work for the purposes of comment and criticism; as an educator, you have a good case that this is your purpose.

However, even then, you should be careful, especially if you're reaching a wide audience with your replication.

So, in summary:

• avoid charges of plagiarism by using appropriate forms of citation
• avoid charges of breaching copyright by understanding your local fair use exceptions.