To me graphoid output looks blurry. Especially the font-rendering of legends.
Here's an example: https://en.wikipedia.org/api/rest_v1/page/graph/png/User%3ABawolff%2Fsandbox/0/966bfb10e60fb463fe1885f50c259ba3b98b3139.png
Is there anything we can do about that? Render in high dpi? Adjust font hinting parameters? Add unsharp mask as a post-processing step?
I think part of the problem is the grey color used combined with the small size.