fix an issue iwth z is forwarded

ultraplot/axes/plot.py

@@ -7632,9 +7632,16 @@ def tripcolor(self, *args, **kwargs):
         # Update kwargs and handle cmap
         kw.update(_pop_props(kw, "collection"))
 
+        # NOTE: Matplotlib colors either the vertices, from the positional z, or
+        # the faces, from the 'facecolors' keyword. Since the input parser pads
+        # missing positional arguments with None, forwarding a null z would make
+        # matplotlib warn that the positional parameter has no effect.
+        values = z
+        if z is None:
+            values = kw.get("facecolors", None)
         center_levels = kw.pop("center_levels", None)
         kw = self._parse_cmap(
-            triangulation.x, triangulation.y, z, center_levels=center_levels, **kw
+            triangulation.x, triangulation.y, values, center_levels=center_levels, **kw
         )
 
         # Handle patch edges, labels, and guide parameters
@@ -7643,8 +7650,9 @@ def tripcolor(self, *args, **kwargs):
         guide_kw = _pop_params(kw, self._update_guide)
 
         # Plot with the native tripcolor method
+        zs = () if z is None else (z,)
         with self._keep_grid_bools():
-            m = self._call_native("tripcolor", triangulation, z, **kw)
+            m = self._call_native("tripcolor", triangulation, *zs, **kw)
 
         # Fix edges and add labels
         self._fix_patch_edges(m, **edgefix_kw, **kw)

ultraplot/tests/test_2dplots.py

@@ -396,6 +396,63 @@ def test_tricontour_levels_respect_explicit_vmin_vmax():
     assert m.norm(0.5) == pytest.approx(0.625)
 
 
+def _triangulation():
+    """
+    Return a two-triangle unit square, for the tripcolor tests below.
+    """
+    from matplotlib.tri import Triangulation
+
+    return Triangulation(
+        [0.0, 1.0, 0.0, 1.0],
+        [0.0, 0.0, 1.0, 1.0],
+        [[0, 1, 2], [1, 3, 2]],
+    )
+
+
+def test_tripcolor_facecolors_without_z():
+    """
+    Coloring the faces needs no z, and must not warn about a positional c.
+
+    The input parser pads absent positional arguments with None, and forwarding
+    that null z made matplotlib warn that the positional parameter had no
+    effect. See the tripcolor override in `ultraplot.axes.plot`.
+    """
+    facecolors = np.array([1.0, 2.0])
+    _, ax = uplt.subplots()
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", UserWarning)
+        m = ax.tripcolor(_triangulation(), facecolors=facecolors)
+    assert np.array_equal(m.get_array(), facecolors)
+    assert m.get_clim() == pytest.approx((1.0, 2.0))
+
+
+def test_tripcolor_vertex_values_unaffected():
+    """
+    Passing z still colors the vertices, and still sets the color limits.
+    """
+    z = np.array([0.0, 1.0, 2.0, 3.0])
+    _, ax = uplt.subplots()
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", UserWarning)
+        m = ax.tripcolor(_triangulation(), z)
+    vmin, vmax = m.get_clim()
+    assert vmin == pytest.approx(0.0)
+    assert vmax == pytest.approx(3.0, abs=1e-6)
+
+
+def test_tripcolor_warns_when_z_and_facecolors_given():
+    """
+    Supplying both is a real conflict: matplotlib discards z, and says so.
+    """
+    _, ax = uplt.subplots()
+    with pytest.warns(UserWarning, match="Positional parameter c has no effect"):
+        ax.tripcolor(
+            _triangulation(),
+            np.array([0.0, 1.0, 2.0, 3.0]),
+            facecolors=np.array([1.0, 2.0]),
+        )
+
+
 def test_tricontour_explicit_colors_match_levels():
     """
     Explicit triangular contour colors should map one-to-one with levels.