There are several problems with your question:
More emphasis is given to the programming language than the design features. Why is Ruby important? If there are better alternatives in another programming language, why not use them? How does programming language relate to design?
It barely matches any of what kind of questions can I ask? bullet points from the FAQ. It might be included in the "Digital Media" bullet, but that's sort of weak on its own since you're asking about the programming language and not an application.
Your design requirements are vague. You ask for something "as visually stimulating as Nodebox or Processing". I can look online for screenshots of implementations, but this still won't tell me what you consider "visually stimulating".
Your question answers the title. It sounds like you've found a product that meets your technical needs. So, "Is there a Ruby datavis platform?"... yes, there is.
Your current shortfall is vague. You state that Graphviz "looks too 1998". "1998" means what in this context? Win98 beige buttons? Geocities? <blink> tags?
Your research isn't well displayed. You advise you found a potential solution, but then only looked at the gallery images. If it meets the technical needs you've listed, why haven't you tried it yet to see for yourself?
In general, it just feels like a shopping question and one that places more emphasis on programming than design. I think it would be justified to close as Off Topic, but I'd rather see the community's reaction than mod intervention.
On a related note, there has been a proposed FAQ update posted on this meta for a week now. Please weigh in with your opinion. GD is a community-driven site, so it's important that the community participates in the meta discussions that affect the main site.