Creating Pie chart for aggregate data

Hi,
I am creating a report with a pie chart using BIRT and currently stuck with this problem. I am using a XML file as the DataSource and it looks like the one given below.

<Disk>
<Name>10K</Name>
<locatedSpace>5.46 GB</locatedSpace>
<FreeSpace>4.69 GB</FreeSpace>
</Disk>

<Disk>
<Name>Legacy SAN</Name>
<locatedSpace>2.46 GB</locatedSpace>
<FreeSpace>5.2 GB</FreeSpace>
</Disk>

I need to create a pie chart that represents the sum(locatedSpace) and sum(Free_Space) in the same chart as sectors and group the chart by the Name. How do I achieve this? I created two column bindings that represent the sum of locatedSpace and the sum of FreeSpace. But the 'Slice Size Definition' in the 'Select Data' tab accepts only column. I am not sure how to proceed after this. Report should look the one in the image attached with this message. Thanks in advance.