draw←{                                      ⍝ Draw over '*'s.
    ⎕IO←1 ⋄ ⍺←'*'                           ⍝ index origin, default marker
    rh(×/¯1↓⍴⍵),¯1↑1,⍴⍵                    ⍝ result shape
    ch←'│─┌─┐─┬││└├┘┤┴┼'                    ⍝ box drawing characters
    z←,' '⍪(' ',(rh⍴⍵),' ')⍪' '             ⍝ matricise, add blank frame
    bvz∊⍺ ⋄ ix←⍸bv                         ⍝ hit indices
    in←{2⊥bv[((-⍵),¯1 1,⍵)∘.+ix]}2+2⊃rh     ⍝ moves ↑,←,→ and ↓
    z[(×in)/ix]←ch[in~0]                    ⍝ replace with approp char
    (⍴⍵)⍴1 1↓¯1 ¯1↓(rh+2)z                 ⍝ remove frame, original shape
}
code_colours

test script

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