I've been looking into the exact same thing, and unfortunately I never found anything better than JFreeChart... :( It's a big downside that you cannot drag and rearrange the tasks.
There are plenty other options, but they all have hefty licensing fees.
Nebula GANTT Chart Widget could be useful if you're developing a program based on the eclipse platform. But we're developing on the netbeans platform, and I think it would be a hassle to port it..
Some other opions I looked into;
SwiftGantt, it fell on that it seemed to be outdated, and not very well used.
These all fell on that the license was too expensive. We wanted open source...