Fix set_ylim unit handling (matplotlib#12468)

* Refactor tests for units

Allows re-using the code setting up a unit converter interface for
Quantity.

* Fix missing call to _process_unit_info in set_ylim

Without it, calling set_ylim using unit support on a blank plot results
in trying to convert None.

Author: dopplershift

lib/matplotlib/axes/_base.py

@@ -3480,6 +3480,7 @@ def set_ylim(self, bottom=None, top=None, emit=True, auto=False,
                 raise TypeError('Cannot pass both `ymax` and `top`')
             top = ymax
 
+        self._process_unit_info(ydata=(bottom, top))
         bottom = self._validate_converted_limits(bottom, self.convert_yunits)
         top = self._validate_converted_limits(top, self.convert_yunits)
 

lib/matplotlib/tests/test_units.py

@@ -5,6 +5,7 @@
 import matplotlib.units as munits
 import numpy as np
 import platform
+import pytest
 
 
 # Basic class that wraps numpy array and has units
@@ -36,12 +37,8 @@ def __array__(self):
         return np.asarray(self.magnitude)
 
 
-# Tests that the conversion machinery works properly for classes that
-# work as a facade over numpy arrays (like pint)
-@image_comparison(baseline_images=['plot_pint'],
-                  tol={'aarch64': 0.02}.get(platform.machine(), 0.0),
-                  extensions=['png'], remove_text=False, style='mpl20')
-def test_numpy_facade():
+@pytest.fixture
+def quantity_converter():
     # Create an instance of the conversion interface and
     # mock so we can check methods called
     qc = munits.ConversionInterface()
@@ -58,12 +55,29 @@ def convert(value, unit, axis):
         else:
             return Quantity(value, axis.get_units()).to(unit).magnitude
 
+    def default_units(value, axis):
+        if hasattr(value, 'units'):
+            return value.units
+        elif np.iterable(value):
+            for v in value:
+                if hasattr(v, 'units'):
+                    return v.units
+            return None
+
     qc.convert = MagicMock(side_effect=convert)
     qc.axisinfo = MagicMock(side_effect=lambda u, a: munits.AxisInfo(label=u))
-    qc.default_units = MagicMock(side_effect=lambda x, a: x.units)
+    qc.default_units = MagicMock(side_effect=default_units)
+    return qc
 
+
+# Tests that the conversion machinery works properly for classes that
+# work as a facade over numpy arrays (like pint)
+@image_comparison(baseline_images=['plot_pint'],
+                  tol={'aarch64': 0.02}.get(platform.machine(), 0.0),
+                  extensions=['png'], remove_text=False, style='mpl20')
+def test_numpy_facade(quantity_converter):
     # Register the class
-    munits.registry[Quantity] = qc
+    munits.registry[Quantity] = quantity_converter
 
     # Simple test
     y = Quantity(np.linspace(0, 30), 'miles')
@@ -77,9 +91,9 @@ def convert(value, unit, axis):
     ax.yaxis.set_units('inches')
     ax.xaxis.set_units('seconds')
 
-    assert qc.convert.called
-    assert qc.axisinfo.called
-    assert qc.default_units.called
+    assert quantity_converter.convert.called
+    assert quantity_converter.axisinfo.called
+    assert quantity_converter.default_units.called
 
 
 # Tests gh-8908
@@ -95,6 +109,15 @@ def test_plot_masked_units():
     ax.plot(data_masked_units)
 
 
+def test_empty_set_limits_with_units(quantity_converter):
+    # Register the class
+    munits.registry[Quantity] = quantity_converter
+
+    fig, ax = plt.subplots()
+    ax.set_xlim(Quantity(-1, 'meters'), Quantity(6, 'meters'))
+    ax.set_ylim(Quantity(-1, 'hours'), Quantity(16, 'hours'))
+
+
 @image_comparison(baseline_images=['jpl_bar_units'], extensions=['png'],
                   savefig_kwarg={'dpi': 120}, style='mpl20')
 def test_jpl_bar_units():