Merge pull request matplotlib#25061 from dstansby/twoslope

Ensure TwoSlopeNorm always has two slopes

Author: jklymak

doc/api/next_api_changes/behavior/25061-DS.rst

@@ -0,0 +1,12 @@
+TwoSlopeNorm now auto-expands to always have two slopes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In the case where either ``vmin`` or ``vmax`` are not manually specified
+to `~.TwoSlopeNorm`, and where the data it is scaling is all less than or
+greater than the center point, the limits are now auto-expanded so there
+are two symmetrically sized slopes either side of the center point.
+
+Previously ``vmin`` and ``vmax`` were clipped at the center point, which
+caused issues when displaying color bars.
+
+This does not affect behaviour when ``vmin`` and ``vmax`` are manually
+specified by the user.

lib/matplotlib/colors.py

@@ -1432,13 +1432,17 @@ def vcenter(self, value):
 
     def autoscale_None(self, A):
         """
-        Get vmin and vmax, and then clip at vcenter
+        Get vmin and vmax.
+
+        If vcenter isn't in the range [vmin, vmax], either vmin or vmax
+        is expanded so that vcenter lies in the middle of the modified range
+        [vmin, vmax].
         """
         super().autoscale_None(A)
-        if self.vmin > self.vcenter:
-            self.vmin = self.vcenter
-        if self.vmax < self.vcenter:
-            self.vmax = self.vcenter
+        if self.vmin >= self.vcenter:
+            self.vmin = self.vcenter - (self.vmax - self.vcenter)
+        if self.vmax <= self.vcenter:
+            self.vmax = self.vcenter + (self.vcenter - self.vmin)
 
     def __call__(self, value, clip=None):
         """

lib/matplotlib/tests/test_colors.py

@@ -665,16 +665,16 @@ def test_TwoSlopeNorm_scale():
 def test_TwoSlopeNorm_scaleout_center():
     # test the vmin never goes above vcenter
     norm = mcolors.TwoSlopeNorm(vcenter=0)
-    norm([1, 2, 3, 5])
-    assert norm.vmin == 0
+    norm([0, 1, 2, 3, 5])
+    assert norm.vmin == -5
     assert norm.vmax == 5
 
 
 def test_TwoSlopeNorm_scaleout_center_max():
     # test the vmax never goes below vcenter
     norm = mcolors.TwoSlopeNorm(vcenter=0)
-    norm([-1, -2, -3, -5])
-    assert norm.vmax == 0
+    norm([0, -1, -2, -3, -5])
+    assert norm.vmax == 5
     assert norm.vmin == -5