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: [http://www.research.att.com/~north/graphviz/download.html] and graphvis.org [http://www.graphviz.org/pub/graphviz/]
I just saw that the Graph Template Library is available for Panther as well. [http://infosun.fmi.uni-passau.de/GTL/]