I am impressed by the Sankey diagrams produced with ‘sankeypython’, a new open source project. I first came across this in a discussion forum, where Yannick Copin from France presented his work using matprotlib.
The project is now at Sourceforge. This sourceforge link not working any more, check Matplotlib project instead (thanks to commenter Johannes for pointing out the brioken link), and several Sankey Diagrams in Python samples can be seen there.
The most important functionality is available, such as labels for flows and units. Diagrams have a basic left-to-right orientation, with vertical arrows branching out on both sides. Color changes seem to be realized by combining several systems. There are apparently no node objects where the flows directly hook to (as is common in other tools), but by interlacing differently colored Sankey arrows a process step becomes kind of implicit.
This has only been up on Sourceforge since June 2011, so it will be interesting to follow further development on ‘Sankey Diagrams in Python’. I have added the project to the list of Sankey diagram software.