Merge pull request matplotlib#4317 from caspervdw/imshow-note

DOC : Note about pixel placement in imshow

This as redundant with the comment for `extent` but this makes it easier to find.

Author: tacaswell

lib/matplotlib/axes/_axes.py

@@ -4683,6 +4683,12 @@ def imshow(self, X, cmap=None, norm=None, aspect=None,
         --------
         matshow : Plot a matrix or an array as an image.
 
+        Notes
+        -----
+        Unless *extent* is used, pixel centers will be located at integer
+        coordinates. In other words: the origin will coincide with the center
+        of pixel (0, 0).
+
         Examples
         --------