HI Niftex,
Easiest way to do this is to:
1) Un-suppress Group Footer #2 (The Group for Week)
2) Add a horizontal line that has the same width as the field however, place it on the Group Footer #2
3) If you need the footer to only appear when drilled into, then add a suppression formula to the section with this code:
DrillDownGroupLevel = 0
-Abhilash