fix: handle NaN colors in plotly create_tile

AGENTS.md

@@ -33,7 +33,7 @@ poetry run inv export             # dump db tables to csv/json/html/md/sql
 - **pre-commit excludes** `alembic/` and `notebooks/`
 - Database is **read-only by default** via URI parameter. Alembic needs write access.
 - Adding a property: update model → alembic revision → upgrade → update PropertyMetadata → `inv render-data-docs`
-- Tests use `-n auto` (pytest-xdist). Set `PYTEST_ADDOPTS=""` to override.
+- Tests use `-n auto` (pytest-xdist). Set `PYTEST_ADDOPTS=""` to override. **Use pytest-style tests only** — plain functions with `assert`, no `unittest.TestCase` or class-based tests.
 - CI matrix: 3 OS × 5 Python versions (3.10–3.14). Runs ruff (pre-commit) then pytest.
 - Documentation uses **sphinx-immaterial** theme (fork of sphinx-material). Workaround: `object_description_options` disables `generate_synopses` to avoid a sphinx-immaterial KeyError. If upgrading sphinx-immaterial or Sphinx, try removing that workaround first. Notebooks in `docs/source/notebooks/` are rendered by nbsphinx. Dev notebooks in root `notebooks/` are ignored by pre-commit.
 - Package published to PyPI on tags via trusted publishing (no password needed).

CONTRIBUTING.md

@@ -203,6 +203,26 @@ poetry run inv render-data-docs
 
 Inspect the ``data.rst`` file before committing to check that everything looks correct.
 
+### Writing Tests
+
+Tests must be **pytest-style** — plain functions with `assert`, no `unittest.TestCase` or class-based tests. Use `@pytest.mark.parametrize` for data-driven tests.
+
+```python
+def test_example():
+    result = some_function()
+    assert result == expected
+
+@pytest.mark.parametrize("input,expected", [(1, 2), (3, 4)])
+def test_parametrized(input, expected):
+    assert some_function(input) == expected
+```
+
+Run the full suite before committing:
+
+```bash
+poetry run pytest
+```
+
 ## Styleguides
 ### Commit Messages
 <!-- TODO

docs/source/conf.py

@@ -69,8 +69,6 @@
     "sphinxcontrib.bibtex",
 ]
 
-nbsphinx_allow_errors = True
-
 # sphinxcontrib.bibtex settings
 bibtex_bibfiles = ["references.bib"]
 

mendeleev/vis/plotly.py

@@ -8,19 +8,24 @@
 
 
 def create_tile(
-    element: pd.Series, color: str, x_offset: float = 0.45, y_offset: float = 0.45
+    element: pd.Series,
+    color: str,
+    default_color: str = "#ffffff",
+    x_offset: float = 0.45,
+    y_offset: float = 0.45,
 ) -> Shape:
     """
     Create tile shape
     """
+    fill = element[color] if pd.notna(element[color]) else default_color
     return Shape(
         type="rect",
         x0=element["x"] - x_offset,
         y0=element["y"] - y_offset,
         x1=element["x"] + x_offset,
         y1=element["y"] + y_offset,
-        line=dict(color=element[color]),
-        fillcolor=element[color],
+        line=dict(color=fill),
+        fillcolor=fill,
         opacity=0.8,
     )
 
@@ -92,7 +97,10 @@ def periodic_table_plotly(
         colorby = "attribute_color"
 
     # tiles
-    tiles = [create_tile(row, color=colorby) for _, row in elements.iterrows()]
+    tiles = [
+        create_tile(row, color=colorby, default_color=missing)
+        for _, row in elements.iterrows()
+    ]
     fig.layout["shapes"] += tuple(tiles)
 
     # symbols

tests/test_vis.py

@@ -1,6 +1,8 @@
+import pandas as pd
 import pytest
 from mendeleev.fetch import fetch_table
 from plotly.graph_objects import Figure as PlotlyFigure
+from plotly.graph_objs.layout import Shape
 from bokeh.plotting import figure as BokehFigure
 from mendeleev.vis import create_vis_dataframe, add_tile_coordinates
 from mendeleev.vis import (
@@ -9,6 +11,7 @@
     heatmap,
     periodic_table,
 )
+from mendeleev.vis.plotly import create_tile
 
 
 def test_add_tile_coordinates():
@@ -50,3 +53,44 @@ def test_periodic_table(attribute, colorby, wide_layout, backend):
         assert isinstance(fig, PlotlyFigure)
     elif backend == "bokeh":
         assert isinstance(fig, BokehFigure)
+
+
+def _make_element(color_value):
+    return pd.Series({"x": 5, "y": 3, "color": color_value})
+
+
+def test_create_tile_valid_hex_color():
+    element = _make_element("#ff0000")
+    tile = create_tile(element, color="color")
+    assert isinstance(tile, Shape)
+    assert tile.fillcolor == "#ff0000"
+    assert tile.line["color"] == "#ff0000"
+
+
+def test_create_tile_nan_color_uses_default():
+    element = _make_element(float("nan"))
+    tile = create_tile(element, color="color")
+    assert tile.fillcolor == "#ffffff"
+    assert tile.line["color"] == "#ffffff"
+
+
+def test_create_tile_nan_color_uses_custom_default():
+    element = _make_element(float("nan"))
+    tile = create_tile(element, color="color", default_color="#cccccc")
+    assert tile.fillcolor == "#cccccc"
+    assert tile.line["color"] == "#cccccc"
+
+
+def test_create_tile_none_color_uses_default():
+    element = _make_element(None)
+    tile = create_tile(element, color="color")
+    assert tile.fillcolor == "#ffffff"
+
+
+def test_create_tile_shape_bounds():
+    element = _make_element("#000000")
+    tile = create_tile(element, color="color", x_offset=0.5, y_offset=0.5)
+    assert tile.x0 == 4.5
+    assert tile.y0 == 2.5
+    assert tile.x1 == 5.5
+    assert tile.y1 == 3.5