Formatting grouped horizontal axis of Pivot Chart

Thanks Andy, but adding Year to the grouping means that you cannot group days in lots of 7 (see attachment), and so the data is no longer smoothed. I suppose this would make sense because a new year could fall midway through a 7-day interval.

But after more than 2 decades of development on Office to date and my wallet £200 lighter from its purchase, I'd have hoped that a simple axis formatting isn't too much to ask from Microsoft. Surely there must be some way to achieve my desired result?

Thanks again.