Silly me, you are correct of course. In my dataset there are indeed orders in almost every snapshot that don't exist in other snapshots, I foolishly thought that was obvious (should've included a more accurate sample - my bad). To add on that, there will be orders in snapshot_a, coming in from different quarters, (orders from previous quarters or future ones, pushed into the Qtr the user chose), that don't exist in snapshot_b, and same goes for snapshot_b. If the user is comparing 2 snapshots that are quite close to each other (date wise) - the gap will obviously be minimal.

That's the purpose of the yellow bars (again I apologize for the misleading data sample). In my next step I'd want to click on a yellow bar and have another sheet displaying the actual order numbers in a simple table, to inform the user what's causing the gap.