The C++ part automatically makes this much harder to maintain

Sorry, what?

Name something that's 30 years old that you use that isn't C/C++. I'll wait.

Well written C/C++ are generally far more maintainable than other languages.

Nobody's touched GraphViz in a meaningful way in 20 years and yet it sails through emscripten without problems

A lot of the reason people think maintaining C/C++ is hard is that maintaining other languages is flat out not possible, and they're used to hearing the war stories from C/C++ because everything else just went extinct

The biggest charting libraries from a decade ago were in Visual Basic, Delphi, Flash, Crystal Reports, and Silverlight. Let me know which one of those you'd prefer. I paid Adobe $500 for Flex Charts in 2013 and the platform it's built on doesn't even exist anymore.

GraphViz is from the VMS big iron era.