Charting data inside filemaker is new and pretty primitive.
As far as tests that I have just made seem to show, your pie chart will chart the first value of each group of records unless you use a field such as a calculation field to compute an aggregate value of all the related records. Neither option is what you've asked for here as your wedges are based on each record in the found set of your parent table.
As far as I can determine, charting portal records so that each record in the portal is a different wedge of the pie chart can't be done with filemaker's native charting tool.
You can chart it on a layout based on your portal's table and a button could switch you to this layout or pop the chart up in a new window.