Do you have any plans for other data sources? SQL perhaps? Does DD work with a BCS connection?
The Dashboard Designer supports external lists, so you can connect SQL Server to SharePoint via BCS and visualize its data.
Also, you can use any external web services with help of JavaScript in init handler:
spchart.com/documentation/data-source
So you can prepare a report in SQL Reporting Services and share it via web service:
msdn.microsoft.com/en-us/library/ms152787.aspx
Next, you should retrieve data from the web service in init handler and process it the same way you do it with SharePoint lists.