add container.py and revise the bar command to return a container

fe03e223b8cb07815f403f0b6567ff6f78f10600
commit fe03e223b8cb07815f403f0b6567ff6f78f10600
diff --git a/lib/matplotlib/axes.py b/lib/matplotlib/axes.py
@@ -4212,6 +4212,7 @@ def _get_legend_handles(self):
             handles.extend([c for c in self.collections
                             if isinstance(c, cls)])
 
+        handles.extend([c for c in self.containers])
         return handles