How to update total values in Chart apps?

Sometimes I need to calculate a percent average value from total values, not from the average of the individual values.
For example:
Col1 Col2 Col3
100 2 50%
50 5 10%

150 7 21,4% not 30%

In a grid app with a group by clause I can reclaculate
{sum_groupby_col3}= {sum_groupby_col1}/{sum_groupby_col2}

But, how can I do the same in a Chart App

I’ve tried {sum_col3}= {sum_col1}/{sum_col2}
on footer event but It doesn’t function

Anybody knows how to recalculate this total values?