Line up the Diagonals of a Grid of Text
J, 12 bytes

|./.&amp;.|:&amp;.|.


Defines an anonymous verb.
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Explanation

|./.&amp;.|:&amp;.|.
|.            Reversed
  /.          anti-diagonals
    &amp;.        under
      |:      transpose
        &amp;.    under
          |.  reversal


In J, u &amp;. v (read: u under v) means "v, then u, then inverse of v".
Reversal and transpose are self-inverses, so the program really means "reverse, transpose, extract reversed anti-diagonals, transpose, reverse".

With example input:

abcd
1234
WXYZ


Reverse:

WXYZ
1234
abcd


Transpose:

W1a
X2b
Y3c
Z4d


Extract reversed anti-diagonals (and pad with spaces):

W  
X1 
Y2a
Z3b
4c 
d  


Transpose:

WXYZ4d
 123c 
  ab  


Reverse:

  ab  
 123c 
WXYZ4d