Or, find an alternative service that can do the work, either for free with watermarks or nominal monthly/annual charges; there's a lot of choices out there at an apparently reasonable price.
Alternatively, you could use open source like Image Magick and write a frontend for it, host it on heroku or AWS EB for a few cents/dollars a month. Basically, you'll need to make a callout to generate the charts from any of the above options.
Finally, maybe you could just create SVG images directly in Apex code. While I find that SVG is not the most human-friendly thing to do, once generated, you could store these files as attachments, and when they open in modern browsers, you'll have your chart image rendered.