[BUG] Fix single trace zero division error

docs/reference/changelog.md

@@ -48,6 +48,11 @@ We will make an effort to standardise and document our versioning policy. For no
 
 - Rename `coloralpha` to `opacity` for consistently with Plotly Express and deprecate the old parameter name ({gh-pr}`245`)
 - Rename `linewidth` to `line_width` for consistency with Plotly's API and deprecate the old parameter name ({gh-pr}`253`)
+- Deprecated `colorscale='default'` and `list_all_colorscale_names()` in favour or Plotly Express' `px.colors.named_colorscales()` ({gh-pr}`262`)
+
+### Bug fixes
+
+- Fixed `ZeroDivisionError` for index-based colormodes when specifying single-trace or single-row plots ({gh-pr}`268`)
 
 ### Dependencies
 
@@ -249,7 +254,6 @@ This release contains a number of improvements to the docs, API reference, CI/CD
 - The `colormode='index'` value has been deprecated in favor of `colormode='row-index'`, which provides the same functionality but is more explicit and allows to distinguish between the `'row-index'` and `'trace-index'` modes ({gh-pr}`114`)
 - The `show_annotations` argument has been deprecated in favor of `show_yticklabels` ({gh-pr}`114`)
 - The `get_all_colorscale_names()` function has been deprecated in favor of `list_all_colorscale_names()` ({gh-pr}`114`)
-- Deprecated `colorscale='default'` and `list_all_colorscale_names()` in favour of Plotly Express' `px.colors.named_colorscales()` ({gh-pr}`262`)
 
 ### Features
 

src/ridgeplot/_color/interpolation.py

@@ -115,13 +115,17 @@ def _mul(a: tuple[Numeric, ...], b: tuple[Numeric, ...]) -> tuple[Numeric, ...]:
 
 
 def _interpolate_row_index(ctx: InterpolationContext) -> ColorscaleInterpolants:
+    if ctx.n_rows == 1:
+        return [[0.0] * ctx.n_traces]
     return [
         [((ctx.n_rows - 1) - ith_row) / (ctx.n_rows - 1)] * len(row)
         for ith_row, row in enumerate(ctx.densities)
     ]
 
 
 def _interpolate_trace_index(ctx: InterpolationContext) -> ColorscaleInterpolants:
+    if ctx.n_traces == 1:
+        return [[0.0]]
     ps = []
     ith_trace = 0
     for row in ctx.densities:
@@ -135,7 +139,10 @@ def _interpolate_trace_index(ctx: InterpolationContext) -> ColorscaleInterpolant
 
 def _interpolate_trace_index_row_wise(ctx: InterpolationContext) -> ColorscaleInterpolants:
     return [
-        [((len(row) - 1) - ith_row_trace) / (len(row) - 1) for ith_row_trace in range(len(row))]
+        [
+            ((len(row) - 1) - ith_row_trace) / (len(row) - 1) if len(row) > 1 else 0.0
+            for ith_row_trace in range(len(row))
+        ]
         for row in ctx.densities
     ]
 

tests/unit/color/test_interpolation.py

@@ -6,7 +6,10 @@
 
 from ridgeplot import ridgeplot
 from ridgeplot._color.interpolation import (
+    SOLID_COLORMODE_MAPS,
+    ColorscaleInterpolants,
     InterpolationContext,
+    SolidColormode,
     _interpolate_color,
     _interpolate_mean_means,
     _interpolate_mean_minmax,
@@ -16,7 +19,7 @@
 from ridgeplot._color.utils import to_rgb
 
 if TYPE_CHECKING:
-    from ridgeplot._types import ColorScale
+    from ridgeplot._types import ColorScale, Densities
 
 
 # ==============================================================
@@ -94,6 +97,41 @@ def test_interpolate_mean_means() -> None:
     assert ps == [[0.0], [0.5], [1.0]]
 
 
+_DENSITY_01 = [(0, 1), (1, 2), (2, 1)]
+_DENSITY_02 = [(1, 1), (2, 2), (3, 1)]
+
+_DENSITIES_ONE_TRACE = [[_DENSITY_01]]
+_DENSITIES_ONE_ROW = [[_DENSITY_01, _DENSITY_02]]
+_DENSITIES_ONE_TRACE_PER_ROW = [[_DENSITY_01, _DENSITY_02], [_DENSITY_02]]
+
+
+@pytest.mark.parametrize(
+    ("colormode", "densities", "expected"),
+    [
+        # One trace
+        ("row-index", _DENSITIES_ONE_TRACE, [[0.0]]),
+        ("trace-index", _DENSITIES_ONE_TRACE, [[0.0]]),
+        ("trace-index-row-wise", _DENSITIES_ONE_TRACE, [[0.0]]),
+        # One row
+        ("row-index", _DENSITIES_ONE_ROW, [[0.0, 0.0]]),
+        ("trace-index", _DENSITIES_ONE_ROW, [[1.0, 0.0]]),
+        ("trace-index-row-wise", _DENSITIES_ONE_ROW, [[1.0, 0.0]]),
+        # One trace per row
+        ("row-index", _DENSITIES_ONE_TRACE_PER_ROW, [[1.0, 1.0], [0.0]]),
+        ("trace-index", _DENSITIES_ONE_TRACE_PER_ROW, [[1.0, 0.5], [0.0]]),
+        ("trace-index-row-wise", _DENSITIES_ONE_TRACE_PER_ROW, [[1.0, 0.0], [0.0]]),
+    ],
+)
+def test_index_based_colormodes(
+    colormode: SolidColormode, densities: Densities, expected: ColorscaleInterpolants
+) -> None:
+    """ZeroDivisionError should never be raised, even when there is only one
+    trace, one row, or one trace per row."""
+    interpolate_func = SOLID_COLORMODE_MAPS[colormode]
+    interpolants = interpolate_func(ctx=InterpolationContext.from_densities(densities))
+    assert interpolants == expected
+
+
 # ==============================================================
 # --- _slice_colorscale()
 # ==============================================================