matplotlib · QuLogic · Aug 6, 2021 · Jul 22, 2021 · Jul 23, 2021 · Aug 1, 2021
diff --git a/doc/users/next_whats_new/collection_offsets.rst b/doc/users/next_whats_new/collection_offsets.rst
@@ -0,0 +1,5 @@
+Setting collection offset transform after initialization
+--------------------------------------------------------
+`.collections.Collection.set_offset_transform()` was added.
+
+Previously the offset transform could not be set after initialization. This can be helpful when creating a `.collections.Collection` outside an axes object and later adding it with `.Axes.add_collection()` and settings the offset transform to `.Axes.transData`.
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -62,7 +62,6 @@ class Collection(artist.Artist, cm.ScalarMappable):
     ignoring those that were manually passed in.
     """
     _offsets = np.zeros((0, 2))
-    _transOffset = transforms.IdentityTransform()
     #: Either a list of 3x3 arrays or an Nx3x3 array (representing N
     #: transforms), suitable for the `all_transforms` argument to
     #: `~matplotlib.backend_bases.RendererBase.draw_path_collection`;
@@ -194,20 +193,17 @@ def __init__(self,
         else:
             self._joinstyle = None
 
+        # default to zeros
         self._offsets = np.zeros((1, 2))
-        # save if offsets passed in were none...
-        self._offsetsNone = offsets is None
-        self._uniform_offsets = None
+
         if offsets is not None:
             offsets = np.asanyarray(offsets, float)
             # Broadcast (2,) -> (1, 2) but nothing else.
             if offsets.shape == (2,):
                 offsets = offsets[None, :]
-            if transOffset is not None:
-                self._offsets = offsets
-                self._transOffset = transOffset
-            else:
-                self._uniform_offsets = offsets
+            self._offsets = offsets
+
+        self._transOffset = transOffset
 
         self._path_effects = None
         self.update(kwargs)
@@ -223,11 +219,23 @@ def get_transforms(self):
         return self._transforms
 
     def get_offset_transform(self):
-        t = self._transOffset
-        if (not isinstance(t, transforms.Transform)
-                and hasattr(t, '_as_mpl_transform')):
-            t = t._as_mpl_transform(self.axes)
-        return t
+        """Return the `.Transform` instance used by this artist offset."""
+        if self._transOffset is None:
+            self._transOffset = transforms.IdentityTransform()
+        elif (not isinstance(self._transOffset, transforms.Transform)
+              and hasattr(self._transOffset, '_as_mpl_transform')):
+            self._transOffset = self._transOffset._as_mpl_transform(self.axes)
+        return self._transOffset
+
+    def set_offset_transform(self, transOffset):
+        """
+        Set the artist offset transform.
+
+        Parameters
+        ----------
+        transOffset : `.Transform`
+        """
+        self._transOffset = transOffset
 
     def get_datalim(self, transData):
         # Calculate the data limits and return them as a `.Bbox`.
@@ -248,8 +256,8 @@ def get_datalim(self, transData):
 
         transform = self.get_transform()
         transOffset = self.get_offset_transform()
-        if (not self._offsetsNone and
-                not transOffset.contains_branch(transData)):
+        hasOffsets = np.any(self._offsets)  # True if any non-zero offsets
+        if hasOffsets and not transOffset.contains_branch(transData):
             # if there are offsets but in some coords other than data,
             # then don't use them for autoscaling.
             return transforms.Bbox.null()
@@ -279,7 +287,7 @@ def get_datalim(self, transData):
                     self.get_transforms(),
                     transOffset.transform_non_affine(offsets),
                     transOffset.get_affine().frozen())
-            if not self._offsetsNone:
+            if hasOffsets:
                 # this is for collections that have their paths (shapes)
                 # in physical, axes-relative, or figure-relative units
                 # (i.e. like scatter). We can't uniquely set limits based on
@@ -542,20 +550,12 @@ def set_offsets(self, offsets):
         offsets = np.asanyarray(offsets, float)
         if offsets.shape == (2,):  # Broadcast (2,) -> (1, 2) but nothing else.
             offsets = offsets[None, :]
-        # This decision is based on how they are initialized above in __init__.
-        if self._uniform_offsets is None:
-            self._offsets = offsets
-        else:
-            self._uniform_offsets = offsets
+        self._offsets = offsets
         self.stale = True
 
     def get_offsets(self):
         """Return the offsets for the collection."""
-        # This decision is based on how they are initialized above in __init__.
-        if self._uniform_offsets is None:
-            return self._offsets
-        else:
-            return self._uniform_offsets
+        return self._offsets
 
     def _get_default_linewidth(self):
         # This may be overridden in a subclass.
@@ -1441,9 +1441,6 @@ def set_segments(self, segments):
                 seg = np.asarray(seg, float)
             _segments.append(seg)
 
-        if self._uniform_offsets is not None:
-            _segments = self._add_offsets(_segments)
-
         self._paths = [mpath.Path(_seg) for _seg in _segments]
         self.stale = True
 
@@ -1474,19 +1471,6 @@ def get_segments(self):
 
         return segments
 
-    def _add_offsets(self, segs):
-        offsets = self._uniform_offsets
-        Nsegs = len(segs)
-        Noffs = offsets.shape[0]
-        if Noffs == 1:
-            for i in range(Nsegs):
-                segs[i] = segs[i] + i * offsets
-        else:
-            for i in range(Nsegs):
-                io = i % Noffs
-                segs[i] = segs[i] + offsets[io:io + 1]
-        return segs
-
     def _get_default_linewidth(self):
         return mpl.rcParams['lines.linewidth']
 

diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -1048,3 +1048,34 @@ def test_get_segments():
     readback, = lc.get_segments()
     # these should comeback un-changed!
     assert np.all(segments == readback)
+
+
+def test_set_offsets_late():
+    identity = mtransforms.IdentityTransform()
+    sizes = [2]
+
+    null = mcollections.CircleCollection(sizes=sizes)
+
+    init = mcollections.CircleCollection(sizes=sizes, offsets=(10, 10))
+
+    late = mcollections.CircleCollection(sizes=sizes)
+    late.set_offsets((10, 10))
+
+    # Bbox.__eq__ doesn't compare bounds
+    null_bounds = null.get_datalim(identity).bounds
+    init_bounds = init.get_datalim(identity).bounds
+    late_bounds = late.get_datalim(identity).bounds
+
+    # offsets and transform are applied when set after initialization
+    assert null_bounds != init_bounds
+    assert init_bounds == late_bounds
+
+
+def test_set_offset_transform():
+    skew = mtransforms.Affine2D().skew(2, 2)
+    init = mcollections.Collection([], transOffset=skew)
+
+    late = mcollections.Collection([])
+    late.set_offset_transform(skew)
+
+    assert skew == init.get_offset_transform() == late.get_offset_transform()