This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I suppose that would depend in what format your data is, and your requirements. There are quite a few visualization libraries. Jung, for instance, displays graphs in a variety of ways, and builds the graph from xml files. If it is simply an image, then no tool is really needed.