you would have to have the rst of the report designed as group ed on the same item your pie chart is grouped on and a summary of whatever you are summarizing in the pie chart (like the sum of sales).
Fromt here you can use group sorting to sort your groups in the report asc or desc on that group summary value. This will also sort your pie cahrt legend. If you need the rest of your report to be different you would have to make your pie chart a subreport follwing the same guidelines.