FIX: scale norm of collections when first array is set

Previously, Collections would defer auto-scaling the norm until
draw time. Now the collection is scaled anytime an array is set
and hasn't already been scaled previously.

Author: greglucas

doc/api/next_api_changes/behavior/27347-GL.rst

@@ -0,0 +1,7 @@
+ScalarMappables auto-scale their norm when an array is set
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Collections previously deferred auto-scaling of the norm until draw time.
+This has been changed to scale the norm whenever the first array is set
+to align with the docstring and reduce unexpected behavior when
+accessing the norm before drawing.

lib/matplotlib/cm.py

@@ -536,6 +536,8 @@ def set_array(self, A):
                             "converted to float")
 
         self._A = A
+        if not self.norm.scaled():
+            self.norm.autoscale_None(A)
 
     def get_array(self):
         """

lib/matplotlib/tests/test_collections.py

@@ -292,6 +292,16 @@ def check_segments(coll, positions, linelength, lineoffset, orientation):
         assert segment[1, pos2] == positions[i]
 
 
+def test_collection_norm_autoscale():
+    # norm should be autoscaled when array is set, not deferred to draw time
+    lines = np.arange(24).reshape((4, 3, 2))
+    coll = mcollections.LineCollection(lines, array=np.arange(4))
+    assert coll.norm(2) == 2 / 3
+    # setting a new array shouldn't update the already scaled limits
+    coll.set_array(np.arange(4) + 5)
+    assert coll.norm(2) == 2 / 3
+
+
 def test_null_collection_datalim():
     col = mcollections.PathCollection([])
     col_data_lim = col.get_datalim(mtransforms.IdentityTransform())