Move Axes.collections into hidden children attribute.

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

There was a quite a bit more fallout from this one:
* `streamplot` had to add the arrows after the line collection, so they
  would remain on top. Because `FancyArrowPatch` is created in display
  coordinates, this meant that their limits changed slightly, as adding
  the line collection would have re-adjusted the `Axes.transData`.
* The `test_pre_transform_plotting` image needed to be regenerated
  because of the above limit change. I took this opportunity to clean up
  some of the previous keep-image-the-same hacks.
* The `test_arrow_contains_point` test image now has 'contained' scatter
  points correctly over the arrows.
* Tests in `test_patches.py` needed to be re-ordered slightly as they
  were checking alpha between collections and non-collection artists.
* The `test_fancy` legend images needed to be regenerated because
  errorbars are correctly drawn as a group now.

Author: QuLogic

lib/matplotlib/axes/_base.py

@@ -11,23 +11,23 @@
 import numpy as np
 
 import matplotlib as mpl
-from matplotlib import _api, cbook
+from matplotlib import _api, cbook, docstring
+import matplotlib.artist as martist
+import matplotlib.axis as maxis
 from matplotlib.cbook import _OrderedSet, _check_1d, index_of
-from matplotlib import docstring
+import matplotlib.collections as mcoll
 import matplotlib.colors as mcolors
+import matplotlib.font_manager as font_manager
+import matplotlib.image as mimage
 import matplotlib.lines as mlines
 import matplotlib.patches as mpatches
-import matplotlib.artist as martist
-import matplotlib.transforms as mtransforms
-import matplotlib.ticker as mticker
-import matplotlib.axis as maxis
+import matplotlib.path as mpath
+from matplotlib.rcsetup import cycler, validate_axisbelow
 import matplotlib.spines as mspines
-import matplotlib.font_manager as font_manager
 import matplotlib.table as mtable
 import matplotlib.text as mtext
-import matplotlib.image as mimage
-import matplotlib.path as mpath
-from matplotlib.rcsetup import cycler, validate_axisbelow
+import matplotlib.ticker as mticker
+import matplotlib.transforms as mtransforms
 
 _log = logging.getLogger(__name__)
 
@@ -1238,7 +1238,6 @@ def cla(self):
         self._current_image = None  # strictly for pyplot via _sci, _gci
         self._projection_init = None  # strictly for pyplot.subplot
         self.legend_ = None
-        self.collections = []  # collection.Collection instances
         self.containers = []
 
         self.grid(False)  # Disable grid on init to use rcParameter
@@ -1306,6 +1305,11 @@ def cla(self):
 
         self.stale = True
 
+    @property
+    def collections(self):
+        return tuple(a for a in self._children
+                     if isinstance(a, mcoll.Collection))
+
     @property
     def images(self):
         return tuple(a for a in self._children
@@ -2069,13 +2073,13 @@ def add_child_axes(self, ax):
 
     def add_collection(self, collection, autolim=True):
         """
-        Add a `~.Collection` to the axes' collections; return the collection.
+        Add a `~.Collection` to the Axes; return the collection.
         """
         label = collection.get_label()
         if not label:
-            collection.set_label('_collection%d' % len(self.collections))
-        self.collections.append(collection)
-        collection._remove_method = self.collections.remove
+            collection.set_label(f'_child{len(self._children)}')
+        self._children.append(collection)
+        collection._remove_method = self._children.remove
         self._set_artist_props(collection)
 
         if collection.get_clip_path() is None:
@@ -4346,7 +4350,6 @@ def format_deltas(key, dx, dy):
     def get_children(self):
         # docstring inherited.
         return [
-            *self.collections,
             *self._children,
             *self.artists,
             *self.spines.values(),

lib/matplotlib/streamplot.py

@@ -210,7 +210,6 @@ def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
 
         p = patches.FancyArrowPatch(
             arrow_tail, arrow_head, transform=transform, **arrow_kw)
-        axes.add_patch(p)
         arrows.append(p)
 
     lc = mcollections.LineCollection(
@@ -222,9 +221,13 @@ def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
         lc.set_cmap(cmap)
         lc.set_norm(norm)
     axes.add_collection(lc)
-    axes.autoscale_view()
 
     ac = matplotlib.collections.PatchCollection(arrows)
+    # Adding the collection itself is broken; see #2341.
+    for p in arrows:
+        axes.add_patch(p)
+
+    axes.autoscale_view()
     stream_container = StreamplotSet(lc, ac)
     return stream_container