Issue with Bar chart

Hello,

We have one variable Material Group = =If(([Mat] InList ("A";"B"));"GroupA";If(([Mat] InList ("C";"D"));"GroupB"))

When we have material A, B, C,D then displying column chart with two bars for GroupA and GroupB (Chart is on Group and Qty Order)

When we have material A, B then bar chart is displaying only with GroupA on X axis as there are no material for GroupB.

But i am expecting both values GroupA and GroupB on X axis. Display one bar for GroupA and no bar for GroupB.

Any sugession please?

Thank you,