Question for macro experts and macro lens owners

I've recently bought Nikon 40mm f2.8 macro lens, and have a question about focus zone when shooting closeups. My camera is Nikon D5100.
As you can see in these pics, the focus stretch is not horizontal or vertical, but a bit angular. I've marked the areas with black lines.

Aperture: 3.2

Original pic: https://picasaweb.google.com/105480313862500361439/Temp#5808426406007805218

Aperture: 3.3

Original pic: https://picasaweb.google.com/105480313862500361439/Temp#5808426273309478130

Aperture: 9.0

Original pic: https://picasaweb.google.com/105480313862500361439/Temp#5808425935806755586

Is this normal with macro lenses?

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My photoblog: http://hym-pictures.blogspot.com