BUG: calculate colorbar boundaries correctly from values (#13114)

efiring · timhoffm · commit a2e251a0e85b · 2019-01-06T11:47:59.000+01:00
Closes #13098. The unusual case of a call to colorbar with specified values occurs in test_colorbar.py test_colorbar_closed_patch. The effect of the bug there was to disable the ticks; but evidently this was considered a feature, because the example image was made with the bug in place, hence with no ticks on the colorbars. Therefore I have modified the test code to explicitly disable the ticks, so the test images don't have to be changed.
diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
@@ -791,7 +791,7 @@ def _process_values(self, b=None):
             self._values = np.array(self.values)
             if self.boundaries is None:
                 b = np.zeros(len(self.values) + 1)
-                b[1:-1] = 0.5 * (self._values[:-1] - self._values[1:])
+                b[1:-1] = 0.5 * (self._values[:-1] + self._values[1:])
                 b[0] = 2.0 * b[1] - b[2]
                 b[-1] = 2.0 * b[-2] - b[-3]
                 self._boundaries = b
diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
@@ -242,17 +242,21 @@ def test_colorbar_closed_patch():
     cmap = get_cmap("RdBu", lut=5)
 
     im = ax1.pcolormesh(np.linspace(0, 10, 16).reshape((4, 4)), cmap=cmap)
+
+    # The use of a "values" kwarg here is unusual.  It works only
+    # because it is matched to the data range in the image and to
+    # the number of colors in the LUT.
     values = np.linspace(0, 10, 5)
+    cbar_kw = dict(cmap=cmap, orientation='horizontal', values=values,
+                   ticks=[])
 
+    # The wide line is to show that the closed path is being handled
+    # correctly.  See PR #4186.
     with rc_context({'axes.linewidth': 16}):
-        plt.colorbar(im, cax=ax2, cmap=cmap, orientation='horizontal',
-                     extend='both', extendfrac=0.5, values=values)
-        plt.colorbar(im, cax=ax3, cmap=cmap, orientation='horizontal',
-                     extend='both', values=values)
-        plt.colorbar(im, cax=ax4, cmap=cmap, orientation='horizontal',
-                     extend='both', extendrect=True, values=values)
-        plt.colorbar(im, cax=ax5, cmap=cmap, orientation='horizontal',
-                     extend='neither', values=values)
+        plt.colorbar(im, cax=ax2, extend='both', extendfrac=0.5, **cbar_kw)
+        plt.colorbar(im, cax=ax3, extend='both', **cbar_kw)
+        plt.colorbar(im, cax=ax4, extend='both', extendrect=True, **cbar_kw)
+        plt.colorbar(im, cax=ax5, extend='neither', **cbar_kw)
 
 
 def test_colorbar_ticks():
