Thanks for the tip. But I ended up taking a different approach, looping through each value in the list:
For( j = 1, j <= N Items(ListofColumns), j++, //step through the row numbers of interest
WinHandle3 << append (Control Chart( //append the control chart to existing window
Sample Label( :Lot_Number ), // using legacy Control Chart platform allows sample label use
Group Size( 1 ),
KSigma( 3 ),
); // close the for loop.
I just appended each chart to an existing window. I also changed to using the legacy Control Chart platform instead of the (improved?) Control Chart builder, because I wanted to specify the sample label instead of a meaningless sequential number on the X-axis. The script above seems to work.