Interactive Visualizations for Trees and Graphs (Bongshin Lee, Ph.D., 2006). See also TreePlus: Interactive Exploration of Networks with Enhanced Tree Layouts, by Bongshin Lee, et al., IEEE Transactions on Visualization and Computer Graphics, 2016.
Graph Data Conversion and Tree Visualization (Srinivas Reddy Guduru, MSc, Dec. 2011, North Dakota State University)
Gephi's Open Graph Viz Platform; see their supported graph formats