Here is a workaround (not pretty, but worked for me 😉 )
When loading the table with the PO Date, you need to add identifier to each record. I used 1 and 2. Whenever the PO Date changes, change the identifier.
Something like this
PO Date PO Doc Identifier 7/29/2002 3005000012 1 7/29/2002 3005000011 1 7/29/2002 3005000010 1 7/29/2002 3005000334 1 7/23/2002 3005000324 2 7/22/2002 3005000002 1 7/22/2002 3005000001 1 7/22/2002 3005000000 1
Now you can add the following expression to all the Background Color options in each dimension and expression.
=if(avg(Identifier)=1,lightgray(),white())
The Background Color options can be found by clicking the plus signs in front of each dimension and expression.