FIX: Handle no-offsets in collection datalim

This adds a check for whether the collection has offsets
passed in initially. This is necessary to handle the (0, 0) offset
case that is desired and distinguishing that from the initial empty
offset of (0, 0). In particular, this is necessary for the
non-transData IdentityTransform passed in during tight_layout calculations.

Author: greglucas

lib/matplotlib/collections.py

@@ -195,8 +195,9 @@ def __init__(self,
 
         # default to zeros
         self._offsets = np.zeros((1, 2))
+        self._has_offsets = offsets is not None
 
-        if offsets is not None:
+        if self._has_offsets:
             offsets = np.asanyarray(offsets, float)
             # Broadcast (2,) -> (1, 2) but nothing else.
             if offsets.shape == (2,):
@@ -278,7 +279,7 @@ def get_datalim(self, transData):
             offsets = offsets.filled(np.nan)
             # get_path_collection_extents handles nan but not masked arrays
 
-        if len(paths) and len(offsets):
+        if len(paths):
             if any(transform.contains_branch_seperately(transData)):
                 # collections that are just in data units (like quiver)
                 # can properly have the axes limits set by their shape +
@@ -290,18 +291,19 @@ def get_datalim(self, transData):
                     offset_trf.transform_non_affine(offsets),
                     offset_trf.get_affine().frozen())
 
-            # 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
-            # those shapes, so we just set the limits based on their
-            # location.
-            offsets = (offset_trf - transData).transform(offsets)
-            # note A-B means A B^{-1}
-            offsets = np.ma.masked_invalid(offsets)
-            if not offsets.mask.all():
-                bbox = transforms.Bbox.null()
-                bbox.update_from_data_xy(offsets)
-                return bbox
+            if self._has_offsets:
+                # 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
+                # those shapes, so we just set the limits based on their
+                # location.
+                offsets = (offset_trf - transData).transform(offsets)
+                # note A-B means A B^{-1}
+                offsets = np.ma.masked_invalid(offsets)
+                if not offsets.mask.all():
+                    bbox = transforms.Bbox.null()
+                    bbox.update_from_data_xy(offsets)
+                    return bbox
         return transforms.Bbox.null()
 
     def get_window_extent(self, renderer):

lib/matplotlib/tests/test_collections.py

@@ -9,6 +9,7 @@
 import matplotlib.pyplot as plt
 import matplotlib.collections as mcollections
 import matplotlib.colors as mcolors
+import matplotlib.path as mpath
 import matplotlib.transforms as mtransforms
 from matplotlib.collections import (Collection, LineCollection,
                                     EventCollection, PolyCollection)
@@ -291,6 +292,17 @@ def test_null_collection_datalim():
                        mtransforms.Bbox.null().get_points())
 
 
+def test_no_offsets_datalim():
+    # A collection with no offsets and a non transData
+    # transform should return a null bbox
+    ax = plt.axes()
+    coll = mcollections.PathCollection([mpath.Path([(0, 0), (1, 0)])])
+    ax.add_collection(coll)
+    coll_data_lim = coll.get_datalim(mtransforms.IdentityTransform())
+    assert_array_equal(coll_data_lim.get_points(),
+                       mtransforms.Bbox.null().get_points())
+
+
 def test_add_collection():
     # Test if data limits are unchanged by adding an empty collection.
     # GitHub issue #1490, pull #1497.