Nicer range for value histogram of integer images.

When an ImageItem's data has an integer dtype, this patch ensures that
each bin of the LUT histogram contains the same number of integer
values, in order to avoid "spikes" in the histogram that are merely due
to some bins covering more integer values than others.

This commit needs testing (it was rebased from an old commit).

pyqtgraph/graphicsItems/ImageItem.py

@@ -1,3 +1,5 @@
+from __future__ import division
+
 from ..Qt import QtGui, QtCore
 import numpy as np
 import collections
@@ -295,7 +297,15 @@ class ImageItem(GraphicsObject):
         if self.image is None:
             return None,None
         stepData = self.image[::step, ::step]
-        hist = np.histogram(stepData, bins=bins)
+        if not np.iterable(bins):
+            mn = stepData.min()
+            mx = stepData.max()
+            if stepData.dtype.kind in "ui": # unsigned or signed int
+                # we want max - min to be a multiple of nbins
+                range = mn, mn + np.ceil((mx - mn) / bins) * bins
+            else:
+                range = mn, mx
+        hist = np.histogram(stepData, bins=bins, range=range)
         return hist[1][:-1], hist[0]
 
     def setPxMode(self, b):