Fix specifying number of levels with log contour

Author: dstansby

doc/users/next_whats_new/log_contour_levels.rst

@@ -0,0 +1,4 @@
+Maximum levels on log-scaled contour plots are now respected
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When plotting contours with a log norm, passing an integer value to the ``levels``
+argument to cap the maximum number of contour levels now works as intended.

lib/matplotlib/contour.py

@@ -969,7 +969,8 @@ 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.
+        locator is not set and the scale is log or the default
+        locator is used.
 
         We need two more levels for filled contours than for
         line contours, because for the latter we need to specify
@@ -980,7 +981,7 @@ def _autolev(self, N):
         """
         if self.locator is None:
             if self.logscale:
-                self.locator = ticker.LogLocator()
+                self.locator = ticker.LogLocator(numticks=N)
             else:
                 self.locator = ticker.MaxNLocator(N + 1, min_n_ticks=1)
 

lib/matplotlib/tests/test_contour.py

@@ -214,6 +214,24 @@ def test_log_locator_levels():
     assert_array_almost_equal(cb.ax.get_yticks(), c.levels)
 
 
+
+@pytest.mark.parametrize("n_levels", [2, 3, 4, 5, 6])
+def test_lognorm_levels(n_levels):
+    x = np.arange(100) + 1
+    y = np.arange(100) + 1
+    x, y = np.meshgrid(x, y)
+    data = x**2 + y**2
+
+    fig, ax = plt.subplots()
+    im = ax.contour(x, y, data, norm=LogNorm(), levels=n_levels)
+    cbar = fig.colorbar(im, ax=ax)
+
+    levels = im.levels
+    visible_levels = levels[(levels <= data.max()) & (levels >= data.min())]
+    # levels parameter promises "no more than n+1 "nice" contour levels "
+    assert len(visible_levels) <= n_levels + 1
+
+
 @image_comparison(['contour_datetime_axis.png'], style='mpl20')
 def test_contour_datetime_axis():
     fig = plt.figure()