Stacking columns underneath other columns

[Newbie0]

I am trying to stack a column under another column in a report, but cannot figure out how. I tried using STACK and FOLD-LINE, but was probably using them wrong.

Example:
Phase May June July
Planning
Center A 9 9 9
Center B 8 8 8
Testing
Center C 7 7 7

Thank you in advance for your help!