diff --git a/skimage/viewer/plugins/base.py b/skimage/viewer/plugins/base.py
index a92140cb..9a8c2cda 100644
--- a/skimage/viewer/plugins/base.py
+++ b/skimage/viewer/plugins/base.py
@@ -27,16 +27,17 @@ class Plugin(QtGui.QDialog):
     overlay : array
         Image used in measurement/manipulation.
     """
-    def __init__(self, callback, parent=None, height=100, width=400):
-        self._viewer = parent
-        QtGui.QDialog.__init__(self, parent)
+    def __init__(self, image_viewer, callback=None, height=100, width=400):
+        self._viewer = image_viewer
+        QtGui.QDialog.__init__(self, image_viewer)
         self.setWindowTitle('Image Plugin')
         self.layout = QtGui.QGridLayout(self)
         self.resize(width, height)
         self.row = 0
-        self.callback = callback
+        if callback is not None:
+            self.callback = callback
 
-        self.arguments = [parent.original_image]
+        self.arguments = [image_viewer.original_image]
         self.keyword_arguments= {}
 
         self.overlay = self._viewer.overlay
diff --git a/skimage/viewer/plugins/canny.py b/skimage/viewer/plugins/canny.py
index cf417a10..db702fc9 100644
--- a/skimage/viewer/plugins/canny.py
+++ b/skimage/viewer/plugins/canny.py
@@ -4,10 +4,10 @@ from skimage.filter import canny
 
 class CannyPlugin(Plugin):
 
-    def __init__(self, parent, *args, **kwargs):
+    def __init__(self, image_viewer, *args, **kwargs):
         height = kwargs.get('height', 100)
         width = kwargs.get('width', 400)
-        super(CannyPlugin, self).__init__(self.callback, parent=parent,
+        super(CannyPlugin, self).__init__(image_viewer,
                                           width=width, height=height)
         self.add_keyword_argument('sigma', 0.005, 0, self.caller)
         self.add_keyword_argument('low_threshold', 0.255, 0, self.caller)
diff --git a/skimage/viewer/utils/core.py b/skimage/viewer/utils/core.py
index efad540d..f7dc6433 100644
--- a/skimage/viewer/utils/core.py
+++ b/skimage/viewer/utils/core.py
@@ -63,6 +63,8 @@ class LinearColormap(LinearSegmentedColormap):
 
 
 class ClearColormap(LinearColormap):
+    """Color map that varies linearly from alpha = 0 to 1
+    """
     def __init__(self, name, rgb):
         r, g, b = rgb
         cg_speq = {'blue':  [(0.0, b), (1.0, b)],