FIX: Only send one update signal when autoscaling norms

Signal are sent for every vmin/vmax update, when autoscaling we were
changing vmin/vmax to None, then updating them which would cause 4
update signals to be sent. We really don't care about the intermediate
signals and only want a single one sent after all the udpates are complete.

Author: greglucas

lib/matplotlib/colors.py

@@ -1362,8 +1362,12 @@ def inverse(self, value):
 
     def autoscale(self, A):
         """Set *vmin*, *vmax* to min, max of *A*."""
-        self.vmin = self.vmax = None
-        self.autoscale_None(A)
+        with self.callbacks.blocked():
+            # Pause callbacks while we are updating so we only get
+            # a single update signal at the end
+            self.vmin = self.vmax = None
+            self.autoscale_None(A)
+        self._changed()
 
     def autoscale_None(self, A):
         """If vmin or vmax are not set, use the min/max of *A* to set them."""

lib/matplotlib/tests/test_colors.py

@@ -1493,6 +1493,11 @@ def test_norm_callback():
     norm.vmax = 5
     assert increment.call_count == 2
 
+    # We only want autoscale() calls to send out one update signal
+    increment.call_count = 0
+    norm.autoscale([0, 1, 2])
+    assert increment.call_count == 1
+
 
 def test_scalarmappable_norm_update():
     norm = mcolors.Normalize()