Ensure TwoSlopNorm always has two slopes

Author: dstansby

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):
         """