I don't think generating pgfplots graphics counts as a derived work, as long as you are not distributing parts of the pgfplots source code. Otherwise every document that uses pgfplots would automatically be GPL-3.0-or-later, which is probably not what the author intended.
Source: I have a tiny bit of copyright on pgfplots, https://github.com/pgf-tikz/pgfplots/commits?author=hmenke
Hi Henri, thanks for all your work on pgf, and congrats on the recent push on CI etc. The reason I think that GPL-3 would bite is that adding support would require more or less translating the pgfplots source to javascript, so that the app can parse and then correctly render the code, and that would definitely be a derived work. But I think the add-on route should be okay.