Now then Marrow......
I've had a look at your graph there and my concern is what you're reading into the VNT solenoid / boost controller.
What you're seeing there in that graph is what the ECU is calling for.
You will note the little black/grey plastic device dangling around by the radiator with an electrical connector on there and two small black pipes. one traces back to behind the engine, the other goes to a small turbo actuator that is about the same size as a tin of tuna fish - when you spot it you will get what I mean!
There is nothing electrical on the turbo itself, therefore the ECU has no way at all of measuring how open / closed the actual VNT system is, so what the ECU is reporting to you is what the ECU is actually asking for at the solenoid stage. What I am saying is your VNT system could be stuck, although I don't think it will be, or the VNT Solenoid / controller is failing.
The solenoid open and closes to provide a vacuum to the turbo (tuna tin), this has a mechanical arm that then moves the vanes within the turbo. this mechanical arm is operated by the vacuum. If your wife is any good in bed a strong suck on the end of the pipe that leads to the tuna tin should see movement!
you should be able to run an actuator test on it with the engine off, you should get movement of about 10mm in the arm, albeit as the test progresses the vacuum gets weaker as the engine is off.
Having said that, I had this same issue, albeit seemingly not as serious, with my 156 1.9jtd m-jet (same engine, same ECU).
I replaced the MAP sensor, MAF, vacuum lines, almost ended up with a new turbo and my tame local mechanic advised that it was the little vacuum actuator / VNT solenoid that was the problem. He fitted a replacement and calibrated it on his examiner and away I went.
Apparently it does need calibrating though, but that is something of a fierce debate!