The dependency:tree goal of the Maven plugin dependency supports various graphical outputs from the version 2.4 up. This is how you would create a diagram showing all dependencies in the com.example group in the dot format:
mvn dependency:tree -Dincludes=com.example -DappendOutput=true -DoutputType=dot -DoutputFile=/path/to/output.gv
(The output is just a text file with the extension Graphviz gv.)
Note: Thanks to the reader Not Relevant for pointing out the right extension and a typo.