I can’t find anything that definitively explains what the day strain graphic means - most specifically the small white line and the different shades of grey beyond the blue. Plenty of explanation how strain is calculated but not how the graphic works.
I think from some of the info available that at least one of these has something to do with my ‘target’ or it could just be my average (as it seems close to that).
Before I’ve reached that white line the coach is telling me I’ve already reached optimal strain - so I’m not completely clear what the different colours mean. It also doesn’t seem to move each day with recovery.
The Whoop AI says it’s ‘likely’ something to do with activity strain - would prefer a more precise explanation 
It’s much clearer on individual activity strain.