MNT: when clearing an Axes via clear/cla fully detach children

Reset the Axes and Figure of the children to None to help break cycles.

Closes matplotlib#6982

Author: tacaswell

lib/matplotlib/axes/_base.py

@@ -1315,7 +1315,10 @@ def __clear(self):
         self._get_patches_for_fill = _process_plot_var_args(self, 'fill')
 
         self._gridOn = mpl.rcParams['axes.grid']
+        old_children = getattr(self, '_children', [])
         self._children = []
+        for chld in old_children:
+            chld.axes = chld.figure = None
         self._mouseover_set = _OrderedSet()
         self.child_axes = []
         self._current_image = None  # strictly for pyplot via _sci, _gci

lib/matplotlib/tests/test_axes.py

@@ -8359,6 +8359,19 @@ def test_extent_units():
         im.set_extent([2, 12, date_first, date_last], clip=False)
 
 
+def test_cla_clears_chlidren_axes_and_fig():
+    fig, ax = plt.subplots()
+    lines = ax.plot([], [], [], [])
+    img = ax.imshow([[1]])
+    for art in lines + [img]:
+        assert art.axes is ax
+        assert art.figure is fig
+    ax.clear()
+    for art in lines + [img]:
+        assert art.axes is None
+        assert art.figure is None
+
+
 def test_scatter_color_repr_error():
 
     def get_next_color():