FIX: Use mappable data when autoscaling colorbar norm

When switching norms of the mappable we should autoscale the norm
using the data if possible rather than setting it to (0, 1).

Author: greglucas

lib/matplotlib/colorbar.py

@@ -301,11 +301,6 @@ def __init__(self, ax, mappable=None, *, cmap=None,
         if mappable is None:
             mappable = cm.ScalarMappable(norm=norm, cmap=cmap)
 
-        # Ensure the given mappable's norm has appropriate vmin and vmax
-        # set even if mappable.draw has not yet been called.
-        if mappable.get_array() is not None:
-            mappable.autoscale_None()
-
         self.mappable = mappable
         cmap = mappable.cmap
         norm = mappable.norm
@@ -1101,7 +1096,10 @@ def _process_values(self):
             b = np.hstack((b, b[-1] + 1))
 
         # transform from 0-1 to vmin-vmax:
+        if self.mappable.get_array() is not None:
+            self.mappable.autoscale_None()
         if not self.norm.scaled():
+            # If we still aren't scaled after autoscaling, use 0, 1 as default
             self.norm.vmin = 0
             self.norm.vmax = 1
         self.norm.vmin, self.norm.vmax = mtransforms.nonsingular(

lib/matplotlib/tests/test_colorbar.py

@@ -657,6 +657,12 @@ def test_colorbar_scale_reset():
 
     assert cbar.outline.get_edgecolor() == mcolors.to_rgba('red')
 
+    # log scale with no vmin/vmax set should scale to the data if there
+    # is a mappable already associated with the colorbar, not (0, 1)
+    pcm.norm = LogNorm()
+    assert pcm.norm.vmin == z.min()
+    assert pcm.norm.vmax == z.max()
+
 
 def test_colorbar_get_ticks_2():
     plt.rcParams['_internal.classic_mode'] = False