matplotlib · jklymak · Jul 8, 2018 · Jun 28, 2017 · jklymak · Jun 12, 2018
diff --git a/doc/api/next_api_changes/2018-06-09-EF-contour_levels.rst b/doc/api/next_api_changes/2018-06-09-EF-contour_levels.rst
@@ -0,0 +1,16 @@
+Contour color autoscaling improvements
+--------------------------------------
+
+Selection of contour levels is now the same for contour and
+contourf; previously, for contour, levels outside the data range were
+deleted.  (Exception: if no contour levels are found within the
+data range, the `levels` attribute is replaced with a list holding
+only the minimum of the data range.)
+
+When contour is called with levels specified as a target number rather
+than a list, and the 'extend' kwarg is used, the levels are now chosen
+such that some data typically will fall in the extended range.
+
+When contour is called with a `LogNorm` or a `LogLocator`, it will now
+select colors using the geometric mean rather than the arithmetic mean
+of the contour levels.
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -1174,22 +1174,45 @@ def _autolev(self, N):
         """
         Select contour levels to span the data.
 
+        The target number of levels, *N*, is used only when the
+        scale is not log and default locator is used.
+
         We need two more levels for filled contours than for
         line contours, because for the latter we need to specify
         the lower and upper boundary of each range. For example,
         a single contour boundary, say at z = 0, requires only
         one contour line, but two filled regions, and therefore
         three levels to provide boundaries for both regions.
         """
+        self._auto = True
         if self.locator is None:
             if self.logscale:
                 self.locator = ticker.LogLocator()
             else:
                 self.locator = ticker.MaxNLocator(N + 1, min_n_ticks=1)
 
         lev = self.locator.tick_values(self.zmin, self.zmax)
-        self._auto = True
-        return lev
+
+        try:
+            if self.locator._symmetric:
+                return lev
+        except AttributeError:
+            pass
+
+        # Trim excess levels the locator may have supplied.
+        under = np.nonzero(lev < self.zmin)[0]
+        i0 = under[-1] if len(under) else 0
+        over = np.nonzero(lev > self.zmax)[0]
+        i1 = over[0] + 1 if len(over) else len(lev)
+        if self.extend in ('min', 'both'):
+            i0 += 1
+        if self.extend in ('max', 'both'):
+            i1 -= 1
+
+        if i1 - i0 < 3:
+            i0, i1 = 0, len(lev)
+
+        return lev[i0:i1]
 
     def _contour_level_args(self, z, args):
         """
@@ -1220,8 +1243,8 @@ def _contour_level_args(self, z, args):
 
         if not self.filled:
             inside = (self.levels > self.zmin) & (self.levels < self.zmax)
-            self.levels = self.levels[inside]
-            if len(self.levels) == 0:
+            levels_in = self.levels[inside]
+            if len(levels_in) == 0:
                 self.levels = [self.zmin]
                 warnings.warn("No contour levels were found"
                               " within the data range.")
@@ -1246,27 +1269,28 @@ def _process_levels(self):
         # (Colorbar needs this even for line contours.)
         self._levels = list(self.levels)
 
+        if self.logscale:
+            lower, upper = 1e-250, 1e250
+        else:
+            lower, upper = -1e250, 1e250
+
         if self.extend in ('both', 'min'):
-            self._levels.insert(0, min(self.levels[0], self.zmin) - 1)
+            self._levels.insert(0, lower)
         if self.extend in ('both', 'max'):
-            self._levels.append(max(self.levels[-1], self.zmax) + 1)
+            self._levels.append(upper)
         self._levels = np.asarray(self._levels)
 
         if not self.filled:
             self.layers = self.levels
             return
 
-        # layer values are mid-way between levels
-        self.layers = 0.5 * (self._levels[:-1] + self._levels[1:])
-        # ...except that extended layers must be outside the
-        # normed range:
-        if self.extend in ('both', 'min'):
-            if self.logscale:
-                self.layers[0] = 1e-150
-            else:
-                self.layers[0] = -1e150
-        if self.extend in ('both', 'max'):
-            self.layers[-1] = 1e150
+        # Layer values are mid-way between levels in screen space.
+        if self.logscale:
+            # Avoid overflow by taking sqrt before multiplying.
+            self.layers = (np.sqrt(self._levels[:-1])
+                           * np.sqrt(self._levels[1:]))
+        else:
+            self.layers = 0.5 * (self._levels[:-1] + self._levels[1:])
 
     def _process_colors(self):
         """

diff --git a/lib/matplotlib/tests/baseline_images/test_axes/contour_colorbar.pdf b/lib/matplotlib/tests/baseline_images/test_axes/contour_colorbar.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_axes/contour_colorbar.png b/lib/matplotlib/tests/baseline_images/test_axes/contour_colorbar.png