On the weekend, I was working on a paper and wanted to have nice diagrams of J48 trees. However, the default visualization in Weka is anything but great looking. Due to lack of Java libraries, I hacked together a little plugin for the Explorer that allows you to use the GraphViz executable dot to generate and display an image:
You can install a release using Weka’s package manager.
Here is an example J48 tree generated for the UCI dataset iris: