First time this has happened to me. It appeared as though, once I selected Bar chart in any report, I was unable to switch back to Pie chart; the Bar chart would just refresh again. Selecting [Total vs Over Time] produced identical results; same Bar chart presented again.
Workaround seemed to be: log out and back in again. Selecting Pie or Bar is now working as intended again. Also, selecting Pie chart Over Time now operates correctly.