matplotlib
diff --git a/‎lib/matplotlib/axes/_base.py
Copy file name to clipboardExpand all lines: lib/matplotlib/axes/_base.py
+6Lines changed: 6 additions & 0 deletions b/‎lib/matplotlib/axes/_base.py
Copy file name to clipboardExpand all lines: lib/matplotlib/axes/_base.py
+6Lines changed: 6 additions & 0 deletions
diff --git a/‎lib/matplotlib/tests/test_axes.py
Copy file name to clipboardExpand all lines: lib/matplotlib/tests/test_axes.py
+10Lines changed: 10 additions & 0 deletions b/‎lib/matplotlib/tests/test_axes.py
Copy file name to clipboardExpand all lines: lib/matplotlib/tests/test_axes.py
+10Lines changed: 10 additions & 0 deletions
@@ -461,6 +461,12 @@ def _plot_args(self, tup, kwargs, return_kwargs=False):
         for prop_name, val in zip(('linestyle', 'marker', 'color'),
                                   (linestyle, marker, color)):
             if val is not None:
+                if prop_name in kwargs:
+                    _api.warn_external(
+                        f"{prop_name} is redundantly defined by the "
+                        f"'{prop_name}' keyword argument and the fmt sting "
+                        f'("{fmt}"). The keyword argument will take '
+                        "precedence.")
                 kw[prop_name] = val
 
         if len(xy) == 2:
 
@@ -624,6 +624,16 @@ def test_fill_units():
     fig.autofmt_xdate()
 
 
+@pytest.mark.xfail('Warning not yet activated.')
+def test_plot_format_kwarg_redundant():
+    with pytest.warns(UserWarning, match="marker .* redundantly defined"):
+        plt.plot([0], [0], 'o', marker='x')
+    with pytest.warns(UserWarning, match="linestyle .* redundantly defined"):
+        plt.plot([0], [0], '-', linestyle='--')
+    with pytest.warns(UserWarning, match="color .* redundantly defined"):
+        plt.plot([0], [0], 'r', color='blue')
+
+
 @image_comparison(['single_point', 'single_point'])
 def test_single_point():
     # Issue #1796: don't let lines.marker affect the grid