When you create a Pie Chart Widget on the project object you can group by the results by any attribute available at the project object. Additionally you can also group by any picklist that you have created on the project grid.
There is a filtering section also where you can add any project object field to the filter, but the picklists are not available for filtering. Is this expected?
Any supported Clarity release
Picklists are intentionally excluded from the Widget filtering capability
When filtering for picklists use the regular filter section of the view
This is common to any object view and all types of Widgets. Picklists are excluded from Widget filters by design.