Create a straight table chart with dimension ItemNo.
Then two expressions:
= FirstSortedValue(Distinct Date, -Qty)
I added a distinct qualifier, so you get a date back when you have two dates with same Qty for the same ItemNo.
edit: I missed that you may have several entries per date and ItemNo and you need to aggregate.
If you don't want to do that in the script, you can try:
=FirstSortedValue( aggr(Date,ItemNo,Date), aggr(-sum(Qty),ItemNo,Date) )