mpl2.0b3

In [4]: plt.imshow(np.random.randint(-128, 128, dtype=np.int8, size=(3, 3)))
Out[4]: <matplotlib.image.AxesImage at 0x7fe00ba2f0f0>

In [5]: /usr/lib/python3.5/site-packages/matplotlib/colors.py:848: RuntimeWarning: overflow encos
  resdat /= (vmax - vmin)

and a fully black image appears.

Pretty sure this is due to #6700 and can be fixed by restoring the casts of vmin and vmax to a floating type. However that type should be determined as in process_value (i.e., np.float32 if dtype.itemsize <= 2 else np.promote_types(dtype, float) -- see the issue for the discussion).

Not making a PR yet because this (as well as the original #6700) may or may not affect the other Normalize classes too? Perhaps that's something that should be factored out.