AFRs in a diesel will be all over the place.
Constantly changing.
Depending on throttle position, engine speed, engine load, boost etc
Your really just looking for a peak to be sure you're not overfueling.
If you had sustained AFR of 18:1, it would show up as high EGTs.
Reading both in conjunction gives you a read on your tune.
Also, if you were seeing AFRs of 18-20 when you're at full boost, you're probably running too rich.
The screen shot of your gauge above, the graphics shows a rich condition as blue, or green suggesting its giving you a cool running condition. That is true for a gas engine, not true for a diesel. I wonder if the graphic is able to be changed to show rich in orange and red colours for a diesel application?