Dashboard- Headcount to be represented as a % - By Length of Service per Grade

Summary: I am looking to represent length of service per grade as a bar graph showing % of headcount . I am able to achieve this when using stacked 100% or presenting the values as percentage of the row in the pivot query, but I need to represent it just as a stacked bar graph showing the variance per grade.

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When I use the formula ("Worker Assignment"."Head Count" * 100)/SUM("Worker Assignment"."Head Count"), the sum is resulting as the total of the rows instead of subtotal for that particular grade.

How can we achieve sum based on group by Grade?