Add galaxy color-magnitude diagram example

examples/galaxy_color_magnitude.py

@@ -0,0 +1,55 @@
+"""Example galaxy colour-magnitude diagram using plotastro.
+
+The red sequence and blue cloud here are illustrative *synthetic* samples,
+not observed galaxies or fits to survey data.
+
+Run from the repository root:
+    python examples/galaxy_color_magnitude.py
+"""
+
+from pathlib import Path
+
+import matplotlib
+
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+import numpy as np
+import plotastro as pa
+
+
+def main() -> None:
+    rng = np.random.default_rng(42)
+
+    # Simulated SDSS-like absolute r-band magnitudes and g-r colours.
+    m_blue = np.clip(rng.normal(-19.7, 1.15, 650), -23.3, -16.8)
+    m_red = np.clip(rng.normal(-21.0, 0.90, 400), -23.3, -16.8)
+
+    blue_color = 0.47 - 0.035 * (m_blue + 20) + rng.normal(0, 0.13, m_blue.size)
+    red_color = 0.79 - 0.025 * (m_red + 20) + rng.normal(0, 0.07, m_red.size)
+
+    pa.set_style("mnras")
+    fig, ax = pa.subplots()
+    ax.scatter(
+        m_blue, blue_color, s=8, alpha=0.55, color=pa.COLORS["blue"],
+        edgecolors="none", rasterized=True, label="Blue cloud (mock)",
+    )
+    ax.scatter(
+        m_red, red_color, s=8, alpha=0.55, color=pa.COLORS["red"],
+        edgecolors="none", rasterized=True, label="Red sequence (mock)",
+    )
+
+    ax.set_xlabel(r"Absolute magnitude $M_r$ (mag)")
+    ax.set_ylabel(r"Rest-frame colour $(g-r)$ (mag)")
+    ax.set_xlim(-23.5, -16.5)
+    ax.set_ylim(0.0, 1.2)
+    ax.legend(loc="upper right", fontsize=7)
+
+    output = Path(__file__).resolve().parent / "figures" / "galaxy_color_magnitude.png"
+    output.parent.mkdir(parents=True, exist_ok=True)
+    fig.savefig(output, dpi=300, bbox_inches="tight")
+    plt.close(fig)
+    print(f"Saved: {output}")
+
+
+if __name__ == "__main__":
+    main()