Hi,
I am Nallakumar. I have more than 6 years of developing plugins for Office Products.
Firstly, I am not exactly fit into your exact requirement. But, I am proposing another effective way of doing it.
Why can't we do the same plugin for Microsoft Project (MPP) or Excel, which has more pre-defined functionalities for Gannt Chart? I am very good in consuming JIRA APIs. I even did a plugin for Office with JIRA APIs already, but for a different functionalities. And, I am confident that necessary planing activities can be done through MPP/Excel and update it to and fro from JIRA through the plugin via the existing JIRA APIs.
Having said that, I am a MS guy and not JAVA.
Thanks,
Nalla