Fix add_argument.

`arguments` is a list, but I was treating it like a dict.

skimage/viewer/plugins/base.py

@@ -80,7 +80,7 @@ class Plugin(QtGui.QDialog):
 
     def add_argument(self, name, low, high, callback, **kwargs):
         name, slider = self.add_slider(name, low, high, callback, **kwargs)
-        self.arguments[name] = slider
+        self.arguments.append(slider)
 
     def add_keyword_argument(self, name, low, high, callback, **kwargs):
         name, slider = self.add_slider(name, low, high, callback, **kwargs)

skimage/viewer/plugins/canny.py

@@ -1,4 +1,5 @@
 from skimage.filter import canny
+
 from .overlayplugin import OverlayPlugin