Added test and requested alterations

Reset pmin to be 600 hPa instead of 60pct of max pressure
Added test to hit edge case where user specifies kelvin t0

src/metpy/plots/skewt.py

@@ -627,12 +627,13 @@ def plot_mixing_lines(self, w=None, p=None, **kwargs):
 
         # Set pressure range if necessary
         if p is None:
-            # from highest pressure to 60% of highest pressure
+            # from highest pressure to 600 hPa
             pmax = max(self.ax.get_ylim())
-            p = np.linspace(0.6 * pmax, pmax) * self.ax.yaxis.units
+            pmin = units.Quantity(600, 'hPa').to(self.ax.yaxis.units)
+            p = np.linspace(pmin, pmax) * self.ax.yaxis.units
 
-        # Dewpoint contours calculated and converted to xaxis units
-        td = dewpoint(vapor_pressure(p, w)).to(self.ax.xaxis.units)
+        # Dewpoint temperature profiles
+        td = dewpoint(vapor_pressure(p, w))
 
         # Assemble data for plotting
         linedata = [np.vstack((t.m, p.m)).T for t in td]

tests/plots/test_skewt.py

@@ -90,6 +90,28 @@ def test_skewt_adiabat_units():
     return fig
 
 
+@pytest.mark.mpl_image_compare(tolerance=.027, remove_text=False, style='default')
+def test_skewt_adiabat_kelvin_base():
+    """Test adiabats and mixing lines can handle different units."""
+    with matplotlib.rc_context({'axes.autolimit_mode': 'data'}):
+        fig = plt.figure(figsize=(9, 9))
+        skew = SkewT(fig, rotation=45)
+        p = np.linspace(950, 100, 10) * units.hPa
+        t = (np.linspace(18, -30, 10) * units.degC).to(units.degK)
+
+        skew.plot(p, t, 'r')
+
+        # At this point the xaxis is actually degC
+        # Add lines using kelvin base
+        t0 = (np.linspace(-20, 40, 5) * units.degC).to(units.degK)
+        skew.plot_dry_adiabats(t0=t0)
+        # add lines with no units (but using kelvin)
+        t0 = np.linspace(253.15, 313.15, 5)
+        skew.plot_moist_adiabats(t0=t0)
+        skew.plot_mixing_lines()
+
+    return fig
+
 @pytest.mark.mpl_image_compare(tolerance=0. if matplotlib.__version__ >= '3.2' else 30.,
                                remove_text=True, style='default')
 def test_skewt_default_aspect_empty():