Oh, I have misunderstood your first post. Category field is really not on the list (I have probably forgotten to add it here). I'll fix the list in the next version. You can still group by category by right clicking the field and selecting Group by category.
For some reason i can not add category into tree view nodes option in preference menu. I dont know if this is a known issue or problem with my own pvd installation. Please take a look
pvd version: 0.9.9.11 os: win xp sp3 brand new db file.
ps: thanks you very much for this wonderful program.