feat: add support for single-color colormaps with constant mapping behavior

docs/colormaps.md

@@ -83,6 +83,12 @@ to the [`cmap.Colormap`][] constructor; `cmap` refers to these objects collectiv
 
         *(same as `['blue', (0.4, 'green'), (0.8, 'yellow'), 'red']`)*
 
+    A single color yields a constant colormap that returns that color for every
+    value of `x` (i.e. `f(x) = c`).  Any explicit stop position on the single
+    color is ignored, since position is meaningless for a constant mapping.
+
+    - `Colormap(['violet'])` {{ cmap_expr: ['violet'] }}
+
 ### `numpy.ndarray`
 
 A [`numpy.ndarray`][], in one of the following formats:

src/cmap/_colormap.py

@@ -98,7 +98,9 @@ class Colormap:
           specifying the position of the color in the gradient. When using color stops,
           the stop position values should be in the range [0, 1]. If no scalar stop
           positions are given, they will be linearly interpolated between any
-          neighboring stops (or 0-1 if there are no stops).
+          neighboring stops (or 0-1 if there are no stops). A single color (e.g.
+          `["red"]` or `{0.5: "red"}`) yields a constant colormap, where `f(x)`
+          returns that color for every value of `x`.
         - a `dict` mapping scalar values to color-like values: e.g.
           `{0.0: "red", 0.5: (0, 1, 0), 1.0: "#0000FF"}`.
         - a matplotlib-style [segmentdata
@@ -1502,7 +1504,18 @@ def _parse_colorstops(
         _clr_seq = list(val)
 
     if len(_clr_seq) == 1:
-        _clr_seq = [None, _clr_seq[0]]
+        # A single color creates a constant colormap f(x) = c for all x in [0, 1].
+        # We duplicate the color at positions 0 and 1, discarding any explicit
+        # stop position the user may have provided (which is meaningless for a
+        # constant mapping).
+        item = _clr_seq[0]
+        if isinstance(item, (tuple, list)) and len(item) == 2:
+            item = item[1]
+        elif (isinstance(item, (tuple, list)) and len(item) == 5) or (
+            isinstance(item, np.ndarray) and item.shape == (5,)
+        ):
+            item = list(item)[1:]
+        _clr_seq = [item, item]
 
     _positions: list[float | None] = []
     _colors: list[Color] = []

tests/test_colormap.py

@@ -106,6 +106,27 @@ def test_colormap_copy() -> None:
     assert cmap1 != {"1234"}
 
 
+def test_single_color_colormap() -> None:
+    """A single-color Colormap should evaluate to that color for all x in [0, 1].
+
+    Regression test for #139: previously a single color injected an implicit
+    transparent stop at position 0 (creating a transparent-to-color gradient).
+    The new behavior is f(x) = c, represented as two stops at 0 and 1, both
+    with the same color.
+    """
+    expected = ColorStops.parse([(0.0, "violet"), (1.0, "violet")])
+    # any of these single-color forms should produce the same constant colormap
+    for value in (
+        ["violet"],
+        {0.5: "violet"},
+        [(0.5, "violet")],  # explicit position is dropped — meaningless for f(x)=c
+    ):
+        cmap = Colormap(value)
+        assert cmap.color_stops == expected
+        for x in (0.0, 0.25, 0.5, 0.75, 1.0):
+            assert cmap(x) == Color("violet")
+
+
 def test_colormap_errors() -> None:
     with pytest.raises(ValueError, match="Colormap 'bad_string' not found"):
         Colormap("bad_string")