Native pie charts aggregate data across multiple records not across multiple fields in one record.
This might surprise many of you but this can be done using one formula field that contains some JavaScript and uses the OEH (On Error HTML) technique. The basic idea is to write some simple JavaScript that generates the chart and process that JavaScript so that it can be set as the value of an onerror attribute of HTML <img> element. I don't have time at the moment to create a demo but I put it on my to do list for the weekend.
