Forum OpenACS Development: Response to ploting service
I found reportlab quite useful to produce pdf's and allegedly it is able to produce charts as well, so you might want to check that out too (search this forum for reportlab). I would guess that it can output to image formats as well as pdf, but I am not sure.
And then there is still the very simple but working approach of a 1x1 gif that gets extended by the <img size> tags ... I think there was an API to produce bar charts in that way in OpenACS 3.x.