Move Axes.artists into hidden children attribute.

The artists can still be accessed via a read-only property, but now are
combined with collections, images, lines, patches, tables and texts for
sorting and drawing purposes.

Additionally, anything added using `Axes.add_artist` will now appear in
the correct type sublist, and may not appear in `Axes.artists` if it
matches any of the other lists.

There are two test image changes, both of which have to do with `bxp`
and its `patch_artist=True` option. This uses a `Patch` with
`Axes.add_artist` instead of `Axes.plot` and so used to appear in
different order. This change now makes `patch_artist=True` and
`path_artist=False` order the same way, since there's no special
grouping.

Author: QuLogic

lib/matplotlib/axes/_base.py

@@ -1232,7 +1232,6 @@ def cla(self):
 
         self._gridOn = mpl.rcParams['axes.grid']
         self._children = []
-        self.artists = []
         self._mouseover_set = _OrderedSet()
         self.child_axes = []
         self._current_image = None  # strictly for pyplot via _sci, _gci
@@ -1305,6 +1304,14 @@ def cla(self):
 
         self.stale = True
 
+    @property
+    def artists(self):
+        return tuple(
+            a for a in self._children
+            if not isinstance(a, (
+                mcoll.Collection, mimage.AxesImage, mlines.Line2D,
+                mpatches.Patch, mtable.Table, mtext.Text)))
+
     @property
     def collections(self):
         return tuple(a for a in self._children
@@ -2035,7 +2042,7 @@ def has_data(self):
 
     def add_artist(self, a):
         """
-        Add an `~.Artist` to the axes, and return the artist.
+        Add an `~.Artist` to the Axes; return the artist.
 
         Use `add_artist` only for artists for which there is no dedicated
         "add" method; and if necessary, use a method such as `update_datalim`
@@ -2047,8 +2054,8 @@ def add_artist(self, a):
         ``ax.transData``.
         """
         a.axes = self
-        self.artists.append(a)
-        a._remove_method = self.artists.remove
+        self._children.append(a)
+        a._remove_method = self._children.remove
         self._set_artist_props(a)
         a.set_clip_path(self.patch)
         self.stale = True
@@ -4349,7 +4356,6 @@ def get_children(self):
         # docstring inherited.
         return [
             *self._children,
-            *self.artists,
             *self.spines.values(),
             *self._get_axis_list(),
             self.title, self._left_title, self._right_title,

lib/matplotlib/tests/baseline_images/test_axes/bxp_custompatchartist.png

@@ -213,7 +213,6 @@ def _get_legend_handles(self, legend_handler_map=None):
 
     def draw(self, renderer):
 
-        orig_artists = list(self.artists)
         orig_children_len = len(self._children)
 
         if hasattr(self, "get_axes_locator"):
@@ -233,10 +232,9 @@ def draw(self, renderer):
             ax.apply_aspect(rect)
             images, artists = ax.get_images_artists()
             self._children.extend(images)
-            self.artists.extend(artists)
+            self._children.extend(artists)
 
         super().draw(renderer)
-        self.artists = orig_artists
         self._children = self._children[:orig_children_len]
 
     def cla(self):