Add a <generateGraphs>true|false</generateGraphs> parameter to the Dashboard mojo for enabling/disabling graph generation.
Because JFreeChart is based on AWT, it may need graphic libs to be present on the system, or using some special options or framework for dealing with this problem, depending on JDK version, ....
This parameter will allow a quicker integration into projects running on Linux platform, enabling dealing with graphical issues later.