Fix plot.line crash for data of shape (1, N) in _title_for_slice on format_item (#5948)

* 🧪 Add failing test case show casing the error

* 🩹 Fix format_item crash when value array of shape (1, )

* 📚 Added change to whats-new.rst

Author: s-weigand

doc/whats-new.rst

@@ -34,7 +34,8 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
-
+- Fix plot.line crash for data of shape ``(1, N)`` in _title_for_slice on format_item (:pull:`5948`).
+  By `Sebastian Weigand <https://github.com/s-weigand>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/formatting.py

@@ -143,7 +143,7 @@ def format_item(x, timedelta_format=None, quote_strings=True):
     elif isinstance(x, (str, bytes)):
         return repr(x) if quote_strings else x
     elif hasattr(x, "dtype") and np.issubdtype(x.dtype, np.floating):
-        return f"{x:.4}"
+        return f"{x.item():.4}"
     else:
         return str(x)
 

xarray/tests/test_plot.py

@@ -754,6 +754,13 @@ def test_slice_in_title(self):
         title = plt.gca().get_title()
         assert "d = 10.01" == title
 
+    def test_slice_in_title_single_item_array(self):
+        """Edge case for data of shape (1, N) or (N, 1)."""
+        darray = self.darray.expand_dims({"d": np.array([10.009])})
+        darray.plot.line(x="period")
+        title = plt.gca().get_title()
+        assert "d = 10.01" == title
+
 
 class TestPlotStep(PlotTestCase):
     @pytest.fixture(autouse=True)