In chart data region, the values display on the Y-axis is depended on the aggregated values of the field added to the Values area.
Based on your description, it seems that you want to display the percentage values of each series instance in the category group. If so, please try to change the values expression of the value filed with following expression:
(Note: the scope of the SUM function is the category group name)
Then you can specify the number format to “Percentage” in “Vertical Axis Properties” dialogbox.
The following screen shot shows the design structure and preview of a similar report. Please take as reference.