For the Group-by feature, we today have a context menu where you can add tables at any time. You can e.g. add the table "Event Type" twice, making the second a slave of the first. Actually, I can't think of any case where it would make sense to have this table twice! Therefor, I think we should consider having them as options instead in a context menu. So, you want the event types menu on? Check! You want to turn it off? Uncheck!

We need to think about how to do with master/slaves. Today, you remove a table by selecting "Group-by > [None]", which is not easy to understand at all. With the new method, you would remove it by unchecking the checkbox.

With the current method, if you remove a master table, all slaves will be removed too. That doesn't have to be the case in the new design. They could propagate towards the top, to keep the model simple. Yes, reordering the tables are a hassle, but it is a hassle also today...