diff --git a/doc/README.md b/doc/README.md
index 48e26788..015693e1 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -3,3 +3,16 @@ To build docs, run `make` in this directory. `make help` lists all targets.
 
 ## Requirements ##
 Sphinx is needed to build doc. Install with `pip install sphinx`.
+
+## Fixing Warnings ##
+
+- "citation not found: R###"
+  $ cd doc/build; grep -rin R### .
+  There is probably an underscore after the reference (e.g. [1]_)
+
+- "Duplicate citation R###, other instance in...""
+  There is probably a [2] without a [1] in one of
+  the docstrings
+
+- Make sure to use pre-sphinxification paths to images
+  (not the _images directory)
diff --git a/skimage/io/_plugins/pil_plugin.py b/skimage/io/_plugins/pil_plugin.py
index e2108145..db58c3ba 100644
--- a/skimage/io/_plugins/pil_plugin.py
+++ b/skimage/io/_plugins/pil_plugin.py
@@ -260,20 +260,3 @@ def imsave(fname, arr, format_str=None):
 
     img = ndarray_to_pil(arr, format_str=format_str)
     img.save(fname, format=format_str)
-
-
-def imshow(arr):
-    """Display an image, using PIL's default display command.
-
-    Parameters
-    ----------
-    arr : ndarray
-       Image to display.  Images of dtype float are assumed to be in
-       [0, 1].  Images of dtype uint8 are in [0, 255].
-
-    """
-    Image.fromarray(img_as_ubyte(arr)).show()
-
-
-def _app_show():
-    pass
diff --git a/skimage/io/tests/test_plugin.py b/skimage/io/tests/test_plugin.py
index 117e5ab9..00dd5b15 100644
--- a/skimage/io/tests/test_plugin.py
+++ b/skimage/io/tests/test_plugin.py
@@ -93,15 +93,18 @@ def test_available():
 
 
 def test_load_preferred_plugins_all():
-    from skimage.io._plugins import pil_plugin
+    from skimage.io._plugins import pil_plugin, matplotlib_plugin
 
     with protect_preferred_plugins():
-        manage_plugins.preferred_plugins = {'all': ['pil']}
+        manage_plugins.preferred_plugins = {'all': ['pil'],
+                                            'imshow': ['matplotlib']}
         manage_plugins.reset_plugins()
 
-        for plugin_type in ('imread', 'imsave', 'imshow'):
+        for plugin_type in ('imread', 'imsave'):
             plug, func = manage_plugins.plugin_store[plugin_type][0]
             assert func == getattr(pil_plugin, plugin_type)
+        plug, func = manage_plugins.plugin_store['imshow'][0]
+        assert func == getattr(matplotlib_plugin, 'imshow')
 
 
 def test_load_preferred_plugins_imread():