As Piwik documentation suggests it is licensed under GPL, are plugin’s which conform to the API obligated to be Open Sourced back to the community even though they may not contain any Piwik open source code.
Is Piwik available for license under any other Open Source License model.
John Dobson
If you only host the plugins on your site and don’t distribute the plugins, then you have the choice whether or not to open source the code.
If you distribute the plugins (e.g., license for a fee to other piwik users), then you are required to use an open source license if your code is substantively a derivative of an existing plugin. Otherwise, you can select your own license.
Custom mods to the Piwik core follows similar, but more restrictive, guidelines.