Based on the commit history, it looks like protovis has not been touched since August 2010. Has the project been abandoned?
http://gitorious.org/protovis/protovis/commits/master
I'm evaluating javascript charting libraries for my company and would hesitate to recommend one that is no longer being developed.
Protovis is no longer being actively developed; instead, it has been reincarnated as D3.js:
http://mbostock.github.com/d3/
There is an introduction for Protovis users that elaborates on the major differences, and why we decided to develop a new library rather than make incremental improvements.