But controllers don't use the radar as the primary aide-memoire for managing a stack! I used SSR for 30 years and I do not believe there is any way to display all labels all the time clearly in busy airspace because there is not enough room on the radar display! In holding patterns the accuracy of track-keeping is such that often 3, 4 or 5 a/c will be exactly over the same point at the same time. OK. one solution is for one label to be switched off for a second or so to prevent "garbling". but it doesn't solve the problem of how to display everything.
The graphic was interesting - and I'll bet it came as a considerable surprise to the pilots and controllers working in the LTMA!!!