Report - Display rows grouped by filter

I've got a report that has a visual level filter on category and then a custom string. Right now it displays all categories with their corresponding custom strings as rows, but I'd like to group the categories to display the custom strings under each category.

Current Display:

CategoryA: Row 1

CategoryA: Row2

CategoryB: Row1

Desired Display: 

CategoryA

     Row 1,

     Row 2

CategoryB

     Row 1

Does anyone know how to do this?