I have been looking at the published Graph API reports in v1.0 reference.
It would be very helpful to show how the reports were calculated/retrieved.
If the REST API calls cannot be displayed, please show the logical thinking in steps.
For example, to get user activity by Yammer group (BTW: this query does not exist). First look at the group, then get the members and then link the users to groups by group guid.
reportRoot: getYammerDeviceUsageUserCounts - Microsoft Graph v1.0 | Microsoft Docs