Yes indeed, it is some kind of Short Run SPC chart.
The idea behind this is to chart different products on the same chart even tho have different target.
There are different versions of this kind of chart.
* the difference chart abs(Value-Target_prod)
* the Z chart (or Zed not sure), the you chart all products at once no matter they have different target and variation. The ones that you plot (Value-Target_prod)/Stdev_prod on the same chart
Search for SQCpack. Extracted from their manual, it's not advertizing, just their manual looks OK
In Short-run SPC, special control charts allow you to chart differences from a target value. Sometimes the absolute difference from target is plotted. In other cases, a normalized difference from target is plotted.
The idea is that a zero difference from target is good and any deviation away from zero is bad.
A Difference chart is plotting the difference from target in absolute units. A Zed chart takes the difference from target, normalizes this difference in units of sigma, and plots this normalized value as the difference.