The add-on does not collect any kind of data from users. That includes but is not limited to :
login credentials
users identifiers
diagram data (UML markup)
diagram image preview
any kind of data generated by the user, Confluence or any other add-on(s)
usage statistics
The add-on does not use a backend, so there is no data storage.
Diagram data is stored inside two Confluence page attachments.
A text attachment stores diagram markup and a binary attachment stores PNG image of a diagram.