DateTime-specific group descriptors (ChartMonthGroupDescriptor, ChartYearGroupDescriptor, etc.) not working with nullable DateTime field type.

You can work around this issue by using plain ChartGroupDescriptors instead. For example if you wanted to use ChartMonthGroupDescriptor, you can achieve the same effect by exposing the Month field of the DateTime> as a int? read-only property on your business object, and then you will be able to properly group by the newly created integer field.