This is what I have just done on my 7.1.2 MP1.1 v2RU SMP to accomplish this:
1. "Reports > Discovery and Inventory > Inventory > Cross-platform > Operating System".
2. Clone the "Count of Computers by Operating System" report.
3. Select the cloned report and press its Edit button.
4. Views tab, Type = Chart, then select Pir from the dropdown below it.
5. Provided a title.
6. Save Changes.