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The limitation of Set Analysis is just that - it cannot be sensitive to Chart Dimensions, because it's only performed once per chart. So, the way around it is to create a conditional flag in your data that satisfy your Set Analysis condition and use the flag instead of the original condition.
For example, if you want the chart to summarize sales only for some Business Units, you create a conditional flag that corresponds to that condition, and use it in SA:
sum({<_BU_Flag={1}>} Sales)
If this is not answering your question, please post your formula and explain your issue in more details.