I read an article about the similarities and differences between data art and data visualisation a short while back (couple of weeks?), but I really can't seem to find it right now. Mike Bostock's visualisations, though very visually pleasing, always tend towards the functional side of the spectrum, serving a purpose of enhancing the comprehension of concepts or relationships that lie in the data, rather than being designed for purely æsthetic aspirations.

There's obviously a place for both visualisations and art, though, and the line between the two is not always clearly defined.