fix(datashader): handle zero-extent point sets instead of NaN canvas (#724)

A single point (or coincident points / an axis-aligned line) has zero spatial
extent, so _compute_datashader_canvas_params computed factor=0 and then
int(round(width / 0)) raised 'cannot convert float NaN to integer'.

Pad any degenerate axis to a unit window centered on its value before deriving
the canvas scale, so the data lands in a finite canvas and renders. Non-degenerate
extents are passed through unchanged, so normal data is unaffected.

Fixes #724.

Author: timtreis

src/spatialdata_plot/pl/_datashader.py

@@ -716,6 +716,11 @@ def _ax_show_and_transform(
     return im
 
 
+def _pad_degenerate_extent(ext: list[Any]) -> list[Any]:
+    """Pad a zero-width extent to a unit window centered on its value; pass others through."""
+    return [ext[0] - 0.5, ext[1] + 0.5] if ext[1] == ext[0] else ext
+
+
 def _compute_datashader_canvas_params(
     x_ext: list[Any],
     y_ext: list[Any],
@@ -725,6 +730,10 @@ def _compute_datashader_canvas_params(
 
     Shared logic used by both the dask-based and pandas-based entry points.
     """
+    # A zero-width extent (single point, coincident points, axis-aligned line) has no scale to
+    # build a canvas from; pad it so the factor below doesn't divide by zero.
+    x_ext, y_ext = _pad_degenerate_extent(x_ext), _pad_degenerate_extent(y_ext)
+
     # Compute canvas size in pixels, capped at the figure's display resolution.
     # Using np.max ensures the canvas never exceeds display pixels on either axis,
     # preventing pixel-based operations (spread, line_width) from being downscaled

tests/pl/test_render_points.py

@@ -29,6 +29,7 @@
     _build_datashader_color_key,
     _ds_aggregate,
     _ds_shade_categorical,
+    _pad_degenerate_extent,
 )
 from spatialdata_plot.pl.render import _warn_groups_ignored_continuous
 from tests.conftest import DPI, PlotTester, PlotTesterMeta, _viridis_with_under_over, get_standard_RNG
@@ -1255,3 +1256,34 @@ def test_density_defaults_silent_and_force_datashader(sdata_blobs: SpatialData,
     last = list(out.plotting_tree.values())[-1]
     assert (last.density, last.density_how, last.method) == (True, "linear", "datashader")
     assert not any("ignored when density=True" in str(w.message) for w in recwarn.list)
+
+
+# ---------------------------------------------------------------------------
+# Zero-extent datashader canvas (#724)
+# ---------------------------------------------------------------------------
+
+
+def test_pad_degenerate_extent():
+    # Regression test for #724: a zero-width extent expands to a unit window centered on the value,
+    # and a non-degenerate extent is passed through unchanged (so normal data is unaffected).
+    assert _pad_degenerate_extent([5.0, 5.0]) == [4.5, 5.5]
+    assert _pad_degenerate_extent([-3.0, -3.0]) == [-3.5, -2.5]
+    assert _pad_degenerate_extent([0.0, 10.0]) == [0.0, 10.0]
+
+
+@pytest.mark.parametrize(
+    "coords",
+    [
+        [[5.0, 5.0]],
+        [[5.0, 5.0], [5.0, 5.0], [5.0, 5.0]],
+        [[5.0, 0.0], [5.0, 1.0], [5.0, 2.0]],
+    ],
+    ids=["single_point", "coincident_points", "axis_aligned_line"],
+)
+def test_datashader_zero_extent_renders(coords):
+    # Regression test for #724: zero-extent point sets crashed the datashader backend with
+    # "cannot convert float NaN to integer". They must now render without raising.
+    df = pd.DataFrame(np.asarray(coords, dtype=float), columns=["x", "y"])
+    sdata = SpatialData(points={"points": PointsModel.parse(df)})
+    sdata.pl.render_points("points", method="datashader").pl.show()
+    plt.close("all")