Improve MEP22 Tool Searching Structure

lib/matplotlib/backend_managers.py

@@ -485,13 +485,15 @@ def _get_cls_to_instantiate(self, callback_class):
         # Find the class that corresponds to the tool
         if isinstance(callback_class, six.string_types):
             # FIXME: make more complete searching structure
-            if callback_class in globals():
+            if hasattr(self.canvas, 'manager') and \
+               hasattr(self.canvas.manager.backend, callback_class):
+                callback_class = getattr(self.canvas.manager.backend, callback_class)
+            elif callback_class in globals():
                 callback_class = globals()[callback_class]
             else:
                 mod = 'backend_tools'
                 current_module = __import__(mod,
                                             globals(), locals(), [mod], -1)
-
                 callback_class = getattr(current_module, callback_class, False)
         if callable(callback_class):
             return callback_class