Your tick marks are not linear in x or in y. On my monitor, the distance between the first -15 and 0 is close to 1 cm more than the distance between 0 and the second -15.
imshow() does not support nonlinear scales, so your implementation would have to be to figure out which linear positions corresponded to your desired ticks, and xtick() at the linear position, but use xticklabel() to lie about the position labels. If you needed datatips to agree with your labels, more work would have to be done to get datatips to lie for you.
This appears to be a magnetic field, so log scale is not appropriate -- magnetic fields are not symmetric on log scales.
