Reports in CMS 12

In CMS 11 and earlier versions, reports could be created in the same screen as the out of the box reports. This was done via the GuiPlugIn attribute

Out of the box reports

In CMS 12 you can no longer add reports on the same screen as explained in the breaking changes. GuiPlugIn no longer exists.

Instead custom reports should have a new menu item so that they load separately from the out of the box reports. 

https://docs.developers.optimizely.com/content-management-system/docs/breaking-changes-in-content-cloud-cms-12