It slightly depends on your data and how you designed your chart object.
In attached sample, I used a pivot table with two dimensions that span linha 1 to linha 3 and A,B,C.
Then I used one expression:
=sum(Value) / sum(total<F1> Value)
where F1 is the field containing linhas 1 to linhas 3. You need to use total to get the market share, please look into the Manual for the total qualifier with aggregation functions.
Hope this helps,
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