How to get nested floating groups in NSTableView?

I currently have an NSTableView with floating groups enabled. What I want to create are nested groups. By nested I mean the following:

• Month 1
  • week 1
    • day 1
    • day 2
• • week 2
    • day 1
    • day 2
• Month 2
  • week 1
    • day 1
    • day 2
• • week 2
    • day 1
    • day 2

When you scroll through the image I want is that the month group view stays at the top of the view until the next month. Meanwhile, the weekly group view remains under the month group view until the next week. Day is the normal cells that flow in the month and week group rows.