Merge pull request matplotlib#27661 from dstansby/auto-backport-of-pr-27647-on-v3.8.x

Backport of pr 27647 on v3.8.x

Author: tacaswell

lib/matplotlib/tests/test_ticker.py

@@ -1797,3 +1797,24 @@ def test_set_offset_string(formatter):
     assert formatter.get_offset() == ''
     formatter.set_offset_string('mpl')
     assert formatter.get_offset() == 'mpl'
+
+
+def test_minorticks_on_multi_fig():
+    """
+    Turning on minor gridlines in a multi-Axes Figure
+    that contains more than one boxplot and shares the x-axis
+    should not raise an exception.
+    """
+    fig, ax = plt.subplots()
+
+    ax.boxplot(np.arange(10), positions=[0])
+    ax.boxplot(np.arange(10), positions=[0])
+    ax.boxplot(np.arange(10), positions=[1])
+
+    ax.grid(which="major")
+    ax.grid(which="minor")
+    ax.minorticks_on()
+    fig.draw_without_rendering()
+
+    assert ax.get_xgridlines()
+    assert isinstance(ax.xaxis.get_minor_locator(), mpl.ticker.AutoMinorLocator)

lib/matplotlib/ticker.py

@@ -2895,7 +2895,7 @@ def __call__(self):
                                'logarithmic scale')
             return []
 
-        majorlocs = self.axis.get_majorticklocs()
+        majorlocs = np.unique(self.axis.get_majorticklocs())
         try:
             majorstep = majorlocs[1] - majorlocs[0]
         except IndexError: