@eat_pb - welcome to MobileRead.
Thanks for the pointer to that library. As you may be able to tell from the lack of recent activity on this thread, implementing this plugin fell into the "too hard" basket for me which I suspected it would. There were not very many non-commercial Python based graphing libraries, and the ones I looked at had annoying long lists of dependencies (version X of A, version Y of B etc) which all just made it way more hassle than I wanted to deal with. I just wanted to download a zip file and have a go.
I don't recall coming across the qwt library though (my memory sucks) so at first glance it looks promising. The whole packaging drama would still have to be addressed (presumably platform specific C++ dlls) though Kovid did volunteer to assist with that.
Personally I think the demand for the plugin is there and it could add a bit of "glam" to the UI as well. However as I have recently created about 6-12 months of backlog for myself of other after hour development projects (non-calibre related) I don't see myself tackling this in the near future. If someone else is willing to carry the baton for it I would be delighted