Thanks for all the replies!
So far, I have attempted to contact the pchart upstream to ask for
clarification, contacted c-pchart[0] to ask if they know how to get in
touch with upstream pchart (they don't), and contacted Ampache[1] to
alert them to the issue and propose a few workarounds that I can think
of.
Richard, I hope you don't mind me referring to you as a "notable
authority" on the GPL ☺
[
0] https://github.com/szymach/c-pchart/issues/35
[
1] https://github.com/ampache/ampache/issues/1515