When Microsoft refreshed the Windows Phone developer portal in advance of the Mango refresh they added various improvements to reporting, an important one being export to file (XLSX). When you open the exported payout detail reports in Excel and enable editing you'll see a number of warnings against a couple of columns. This is because the data is formatted as text but Excel recognises that the contents are numbers. In order to use these values in formulas or graphs you'll need to convert them to numerical values.
The easiest way I found to do this was to select an empty cell and Copy it, then select the entire columns of "Units Sold" and "Royalty", right click and select "Paste Special". From the popup select "Add" as the operation and click "Ok". Now all your number columns are really numbers (you'll see the formatting change to right alignment).