Prior to ImageMagick 6.5.7-8 (1623), $x was positive when shifting to the left and negative when shifting to the right, and $y was positive when shifting an image up and negative when shifting an image down. Somewhere betwen ImageMagick 6.3.7 (1591) and ImageMagick 6.5.7-8 (1623), the axes of $x and $y were flipped, so that $x was negative when shifting to the left and positive when shifting to the right, and $y was negative when shifting an image up and positive when shifting an image down. Somewhere between ImageMagick 6.5.7-8 (1623) and ImageMagick 6.6.9-7 (1641), the axes of $x and $y were flipped back to pre-ImageMagick 6.5.7-8 (1623) functionality.