Updated March 2011: In the latest version, we have now made it easier to locate the Pie Chart button. Please look at the toolbar above any report. You will see a button “Chart.” Click it and it should show you the pie chart on the currently sorted column in the report.
For example, if you are looking at the Folder Sizes report. Click on the Size column header to sort by that column. Now if you click the Chart button, it will show you a pie chart on “sizes” for the largest N items. Similarly, you can sort on the columns “Number of files” and “Number of folders” to see a chart on those columns.