Backport PR matplotlib#19978: fixed bug in CenteredNorm, issue matplotlib#19972

Author: QuLogic

lib/matplotlib/colors.py

@@ -1341,6 +1341,8 @@ def __init__(self, vcenter=0, halfrange=None, clip=False):
             array([0.25, 0.5 , 1.  ])
         """
         self._vcenter = vcenter
+        self.vmin = None
+        self.vmax = None
         # calling the halfrange setter to set vmin and vmax
         self.halfrange = halfrange
         self.clip = clip
@@ -1364,7 +1366,7 @@ def autoscale(self, A):
     def autoscale_None(self, A):
         """Set *vmin* and *vmax*."""
         A = np.asanyarray(A)
-        if self.vmax is None and A.size:
+        if self._halfrange is None and A.size:
             self.autoscale(A)
 
     @property

lib/matplotlib/tests/test_colors.py

@@ -418,6 +418,12 @@ def test_CenteredNorm():
     norm.autoscale_None([1, 2])
     assert norm.vmax + norm.vmin == 2 * vcenter
 
+    # Check that halfrange can be set without setting vcenter and that it is
+    # not reset through autoscale_None.
+    norm = mcolors.CenteredNorm(halfrange=1.0)
+    norm.autoscale_None([1, 3000])
+    assert norm.halfrange == 1.0
+
     # Check that halfrange input works correctly.
     x = np.random.normal(size=10)
     norm = mcolors.CenteredNorm(vcenter=0.5, halfrange=0.5)