When does "copying" a math diagram become plagiarism? If you copy paste a diagram from a paper/book onto your own work, without permission or attribution, that's plagiarism.
What happens if you see a beautiful diagram about what you want to explain and you replicate it identically without copy pasting?
What if you don't copy it identically but it's still similar (say the colors are different, and the vertices/edges/boxes) have slightly different shapes? For example the original used a 3D blob to explain what a local chart is, and your diagram uses an ellipsoid.
What if you transfer mediums? For example a chalkboard diagram turned into a metafont diagram?
Since there are only so many ways to convey certain mathematical ideas, such as what a bivector is, or how to compute a spline. When do you know where your source of inspiration is too close to your own work that you walked over into 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.