I←{¯1+(≢⍺)↓i⊣i[i]←+\(≢⍺)>i←⍋⍺⍪⍵}

   x ← ↑ 'Fi' 'Jay' 'John' 'Morten' 'Roger'
   y ← ↑ 'JD' 'Jd' 'Geoff' 'Anna' 'Scott' 'Zeus  '

   x I y
0 1 0 ¯1 4 4

   x I x
0 1 2 3 4

¯∞       Fi       Jay      John     Morten   Roger   ∞
↓        ↓        ↓        ↓        ↓        ↓       ↓
(       )[       )[       )[       )[       )[       )
   ¯1        0        1        2        3        4   
  Anna     Fi        Jay      John    Morten   Roger
           JD        Jd                        Scott
           Geoff                               Zeus