Raphaël ( http://raphaeljs.com/ )in combination with the Dracula Graph Library( http://dracula.ameisenbar.de/ ) will get you started.
and also thejit is perfect:
Multi-Trees
The JavaScript InfoVis Toolkit provides tools for creating Interactive Data Visualizations for the Web.
http://thejit.org/
Edit:
you can also take a look at Processing.js
http://processingjs.org/
Processing.js is the sister project of the popular Processing visual programming language, designed for the web. Processing.js makes your data visualizations, digital art, interactive animations, educational graphs, video games, etc. work using web standards and without any plug-ins. You write code using the Processing language, include it in your web page, and Processing.js does the rest.
