This might be clearer:

```
The viewer's ``show()`` method returns a nested set of tuples. Each plugin 
returns two pieces of data: an overlay of the same shape as the input image, 
and a data field which may be ``None`` depending on plugin. Each plugin class 
documents what will be returned as ``data`` in its ``output`` method. The 
first index references the ``(overlay, data)`` pair from a specific plugin. 
The second index, when present, selects either ``overlay`` (0) or ``data`` 
(1). In this example we want both ``overlay`` and ``data`` fields, so the 
second index is omitted. Here, when ``LineProfile`` is used, ``overlay`` 
contains a drawing of the line (including its width), and ``data`` contains 
the measured line profile.
```