Make matplotlib optional dependency for visualization

Core framework now works without matplotlib. Visualization functions
moved to optional 'viz' dependency with graceful fallback.

Install with: pip install 'thurstone[viz]' for visualization features.

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

Author: microprediction

pyproject.toml

@@ -10,7 +10,7 @@ description = "OO toolkit for lattice-based performance models and the horse-rac
 authors = [{name="You", email="you@example.com"}]
 readme = "README.md"
 requires-python = ">=3.9"
-dependencies = ["numpy>=1.23", "matplotlib>=3.7"]
+dependencies = ["numpy>=1.23"]
 
 [project.optional-dependencies]
 test = [
@@ -19,6 +19,9 @@ test = [
     "hypothesis>=6.0",
     "pytest-benchmark>=4.0",
 ]
+viz = [
+    "matplotlib>=3.7",
+]
 
 [tool.setuptools.packages.find]
 where = ["."]

thurstone/visualization.py

@@ -8,9 +8,16 @@
 from __future__ import annotations
 from typing import Tuple, Optional, Dict, Any, List
 import numpy as np
-import matplotlib.pyplot as plt
-from matplotlib.colors import LinearSegmentedColormap
-from mpl_toolkits.mplot3d import Axes3D
+
+# Optional matplotlib import with graceful fallback
+try:
+    import matplotlib.pyplot as plt
+    from matplotlib.colors import LinearSegmentedColormap
+    from mpl_toolkits.mplot3d import Axes3D
+    MATPLOTLIB_AVAILABLE = True
+except ImportError:
+    MATPLOTLIB_AVAILABLE = False
+    plt = None
 
 from .cube_to_simplex import CubeToSimplexMapping
 from .quality_assessment import (
@@ -19,6 +26,15 @@
 )
 
 
+def _ensure_matplotlib():
+    """Ensure matplotlib is available for visualization functions."""
+    if not MATPLOTLIB_AVAILABLE:
+        raise ImportError(
+            "Matplotlib is required for visualization functions. "
+            "Install with: pip install 'thurstone[viz]' or pip install matplotlib"
+        )
+
+
 def create_lattice_grid(resolution: int = 20) -> Tuple[np.ndarray, np.ndarray]:
     """
     Create a regular lattice grid in [0,1]^2.
@@ -49,6 +65,8 @@ def plot_cube_lattice_2d(mapping: CubeToSimplexMapping,
     Returns:
         matplotlib Figure
     """
+    _ensure_matplotlib()
+
     if mapping.k != 2:
         raise ValueError("This function is only for k=2 (triangle)")