I see that the Tablix has a Sorting option in its Properties dialog.
This appears to apply a default sort order to the data in the display when the report is loaded.
I am trying to figure out how to make the column headers clickable in order to implement user-defined sorting. Is this possible in SSRS? If so then how can I go about implementing this? Or is user-defined sorting at the column level implemented in a different way in SSRS?