Protovis is a data visualization tool from the Stanford Visualization Group. We mentioned it last month when we covered the group's Data Wrangler project.
Rubyvis is a port of Protovis, which uses JavaScript, to Ruby. Both use SVG for output.
Here's a simple example from the Rubyvis site:
Rubyvis is open source under the BSD license. It can be found on Github here.
Some more examples:
We covered JavaScript statistics and data visualization libraries today.