Query for 'User Activity' Webi Line Chart using BO4.0 Auditor

Need to create a webi line chart to measure  ‘user activity’ per month, year or day using BO4.0 Auditor on Oracle.    

This question raises two sub-questions:

1. What actions constitute user activity?  Is it concurrent logins by users?   I am confused if that would really give a true picture of user activity as users can log in and do nothing on the BO system.  
2. If concurrent logins do in fact give a true picture of user activity then how do I go about creating a chart? What objects do I need to include in query?

Please provide feedback?

Thank u.