Dynamic column change

Hello All,

I created a column like , for top 50 productnames based on sales productnames  will be there and for rest other products "Others"is assigned(col 1)

and another column  top 50 productnames based on quantity sold,productnames will be there and for rest other products "Others"is assigned(col 2)

and another column  top 50 productnames based on Gross Margin ,productnames will be there and for rest other products "Others"is assigned(Col 3)

Now I created a separate table with SALES,QUANTITY and GROSSMARGIN

if i select SALES ,i want col1 in legend in piechart.if selection is QUANTITY col2 should be legend,if i select GROSSMARGIN col3 should be in legend.

How to achieve that??