[Paraview] Interpretation of histogram of deformation field magnitude?

[motes motes]

I have loaded a deformation field (.mhd file) in paraview made with itk:

1) open .mhd file
2) connect the calculate and use the below expression:

  MetaImage_X*iHat+MetaImage_Y*jHat+0*kHat

3) Add glyph filter.

The result looks like this:

http://37133.vs.webtropia.com/apache2-default/test/deform.png


But now I would like to create a histogram over the magnitude of the
deformation field. I have therefore selected: Filters->Data
Analysis->Histogram.

In the display tab I have selected GlypVector_total (Magnitude) and
under "User Data Array" I have selected MetaImage. The result looks
like this:

http://37133.vs.webtropia.com/apache2-default/test/hist.png

But how should it be interpretated? I assume that the y-axis is the
pixel count and the x-axis is the magnitude size. But should the
magnitudes (Norm) not always be positive?

And as can be seen from the deformation field its only a small part of
the image that is deformed. I would therefore expect a large number of
pixels to have 0 magnitude, but that is not the case.

Any input are welcome!