TEST: Add testing helpers for checking matplotlib version

This encapsulates the proper use of packaging.version.Version to do
version comparisons. Use these instead of remaining direct string comparisons.

src/metpy/testing.py

@@ -10,8 +10,10 @@
 import contextlib
 import functools
 
+import matplotlib
 import numpy as np
 import numpy.testing
+from packaging.version import Version
 from pint import DimensionalityError
 import pytest
 import xarray as xr
@@ -22,6 +24,39 @@
 from .units import units
 
 
+MPL_VERSION = Version(matplotlib.__version__)
+
+
+def mpl_version_before(ver):
+    """Return whether the active matplotlib is before a certain version.
+
+    Parameters
+    ----------
+    ver : str
+        The version string for a certain release
+
+    Returns
+    -------
+        bool : whether the current version was released before the passed in one
+    """
+    return MPL_VERSION < Version(ver)
+
+
+def mpl_version_equal(ver):
+    """Return whether the active matplotlib is equal to a certain version.
+
+    Parameters
+    ----------
+    ver : str
+        The version string for a certain release
+
+    Returns
+    -------
+        bool : whether the current version is equal to the passed in one
+    """
+    return MPL_VERSION == Version(ver)
+
+
 def needs_module(module):
     """Decorate a test function or fixture as requiring a module.
 

tests/plots/test_skewt.py

@@ -11,10 +11,9 @@
 import pytest
 
 from metpy.plots import Hodograph, SkewT
+from metpy.testing import mpl_version_equal
 from metpy.units import units
 
-MPL_VERSION = matplotlib.__version__[:5]
-
 
 @pytest.mark.mpl_image_compare(remove_text=True, style='default', tolerance=0.069)
 def test_skewt_api():
@@ -86,7 +85,7 @@ def test_skewt_default_aspect_empty():
 @pytest.mark.mpl_image_compare(tolerance=0., remove_text=True, style='default')
 def test_skewt_mixing_line_args():
     """Test plot_mixing_lines accepting kwargs for mixing ratio and pressure levels."""
-    # Explicitly pass default values as kwargs the, should recreate NWS SkewT PDF as above
+    # Explicitly pass default values as kwargs, should recreate NWS SkewT PDF as above
     fig = plt.figure(figsize=(12, 9))
     skew = SkewT(fig, rotation=43)
     mlines = np.array([0.0004, 0.001, 0.002, 0.004, 0.007, 0.01, 0.016, 0.024, 0.032])
@@ -156,8 +155,8 @@ def test_skewt_units():
     skew.ax.axvline(-10, color='orange')
 
     # On Matplotlib <= 3.6, ax[hv]line() doesn't trigger unit labels
-    assert skew.ax.get_xlabel() == ('degree_Celsius' if MPL_VERSION == '3.7.0' else '')
-    assert skew.ax.get_ylabel() == ('hectopascal' if MPL_VERSION == '3.7.0' else '')
+    assert skew.ax.get_xlabel() == ('degree_Celsius' if mpl_version_equal('3.7.0') else '')
+    assert skew.ax.get_ylabel() == ('hectopascal' if mpl_version_equal('3.7.0') else '')
 
     # Clear them for the image test
     skew.ax.set_xlabel('')