Correctly extend a lognormed colorbar

lib/matplotlib/colorbar.py

@@ -692,10 +692,18 @@ def _process_values(self, b=None):
                                                                 expander=0.1)
 
             b = self.norm.inverse(self._uniform_y(self.cmap.N + 1))
-            if self._extend_lower():
-                b[0] = b[0] - 1
-            if self._extend_upper():
-                b[-1] = b[-1] + 1
+
+            if isinstance(self.norm, colors.LogNorm):
+                # If using a lognorm, ensure extensions don't go negative
+                if self._extend_lower():
+                    b[0] = 0.9 * b[0]
+                if self._extend_upper():
+                    b[-1] = 1.1 * b[-1]
+            else:
+                if self._extend_lower():
+                    b[0] = b[0] - 1
+                if self._extend_upper():
+                    b[-1] = b[-1] + 1
         self._process_values(b)
 
     def _find_range(self):

lib/matplotlib/tests/test_colorbar.py

@@ -10,7 +10,7 @@
 from matplotlib.testing.decorators import image_comparison, cleanup
 import matplotlib.pyplot as plt
 from matplotlib import rcParams
-from matplotlib.colors import BoundaryNorm
+from matplotlib.colors import BoundaryNorm, LogNorm
 from matplotlib.cm import get_cmap
 from matplotlib import cm
 from matplotlib.colorbar import ColorbarBase
@@ -325,6 +325,15 @@ def test_colorbar_get_ticks():
     assert defTicks.get_ticks().tolist() == levels
 
 
+@cleanup
+def test_colorbar_lognorm_extension():
+    # Test that colorbar with lognorm is extended correctly
+    f, ax = plt.subplots()
+    cb = ColorbarBase(ax, norm=LogNorm(vmin=0.1, vmax=1000.0),
+                      orientation='vertical', extend='both')
+    assert cb._values[0] >= 0.0
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)