BUG: make MaxNLocator.tick_values() obey _symmetric value.

A likely use case is contouring fields such as a velocity component, where one wants to use a diverging colormap centered on zero. Closes #7271.
matplotlib · QuLogic · Oct 18, 2016 · Oct 17, 2016 · Oct 17, 2016 · d4efb18547f241567dfe9720a3c5cfed08c12651
commit d4efb18547f241567dfe9720a3c5cfed08c12651
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
@@ -10,6 +10,7 @@
 from matplotlib.testing.decorators import cleanup, image_comparison
 from matplotlib import pyplot as plt
 from nose.tools import assert_equal, assert_raises
+from numpy.testing import assert_array_almost_equal
 import warnings

 import re
@@ -292,6 +293,15 @@ def test_contourf_decreasing_levels():
    assert_equal(len(w), 2)


+@cleanup
+def test_contourf_symmetric_locator():
+    # github issue 7271
+    z = np.arange(12).reshape((3, 4))
+    locator = plt.MaxNLocator(nbins=4, symmetric=True)
+    cs = plt.contourf(z, locator=locator)
+    assert_array_almost_equal(cs.levels, np.linspace(-12, 12, 5))
+
+
 if __name__ == '__main__':
    import nose
    nose.runmodule(argv=['-s', '--with-doctest'], exit=False)
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1701,9 +1701,13 @@ def __call__(self):
        return self.tick_values(vmin, vmax)

    def tick_values(self, vmin, vmax):
+        if self._symmetric:
+            vmax = max(abs(vmin), abs(vmax))
+            vmin = -vmax
        vmin, vmax = mtransforms.nonsingular(
            vmin, vmax, expander=1e-13, tiny=1e-14)
        locs = self._raw_ticks(vmin, vmax)
+
        prune = self._prune
        if prune == 'lower':
            locs = locs[1:]
@@ -1715,9 +1719,8 @@ def tick_values(self, vmin, vmax):

    def view_limits(self, dmin, dmax):
        if self._symmetric:
-            maxabs = max(abs(dmin), abs(dmax))
-            dmin = -maxabs
-            dmax = maxabs
+            dmax = max(abs(dmin), abs(dmax))
+            dmin = -dmax

        dmin, dmax = mtransforms.nonsingular(
            dmin, dmax, expander=1e-12, tiny=1e-13)