GraphViz
Crowd-sourced documentation and tutorials by the Cocoa developer community
Graphviz (also known as dotty) is a command line tool which takes a simple textual description of a graph and calculates the layout, which can be saved in many different formats (pdf, ps, png, ...).
So basically it is GnuPlot for graphs (and trees).
A Mac version with a GUI is available from PixelGlow http://www.pixelglow.com/graphviz/
The source code is available from AT&T: [1] and graphvis.org [2]
I just saw that the Graph Template Library is available for Panther as well. [3]
