Pole 8, vertically magnetized blocks with +By.
Translate the upper block toward +x by 1 mm.
Translate the lower block toward -x by 1 mm.
This doesn't change the gap, so the x-trajectory is not affected much, if any.

The upper block has negative charges near the gap.
The translation is equivalent to adding negative charges at +x
  and positive charges at -x.
This makes +Bx.

The lower block has plus charges near the gap.
The translation is equivalent to adding positive charges at -x
  and negative charges at +x.
This makes +Bx.

y-slope before pole move: 0
y-slope after pole move: -5.2064e-004
(sign correct for +Bx)

Sensitivity:
translating the upper and lower blocks by 1 mm in opposite directions changes the slope by 5.20e-4