Apple Logic Pro 9 - Manual (Page 934)

To create a barre
Do one of the following:
µ
Drag the left or right edge of an existing dot to the right or left.
µ
Place the pointer over a string, then drag to the left or right.
To move an existing barre
µ
Drag the barre to a new fret position.
934 Chapter 31 Working with Notation