feat(seaborn): implement span-basic (#5611)

## Implementation: `span-basic` - python/seaborn

Implements the **python/seaborn** version of `span-basic`.

**File:** `plots/span-basic/implementations/python/seaborn.py`

**Parent Issue:** #980

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25160803525)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/span-basic/implementations/python/seaborn.py

@@ -1,50 +1,82 @@
-""" pyplots.ai
+""" anyplot.ai
 span-basic: Basic Span Plot (Highlighted Region)
-Library: seaborn 0.13.2 | Python 3.13.11
-Quality: 91/100 | Created: 2025-12-23
+Library: seaborn 0.13.2 | Python 3.13.13
+Quality: 88/100 | Updated: 2026-04-30
 """
 
+import os
+
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
 import seaborn as sns
 
 
-# Data - Monthly sales with recession period and target threshold
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+ELEVATED_BG = "#FFFDF6" if THEME == "light" else "#242420"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+
+BRAND = "#009E73"  # Okabe-Ito position 1 — always first series
+SPAN_RECESSION = "#D55E00"  # Okabe-Ito position 2
+SPAN_TARGET = "#0072B2"  # Okabe-Ito position 3
+
+sns.set_theme(
+    style="ticks",
+    rc={
+        "figure.facecolor": PAGE_BG,
+        "axes.facecolor": PAGE_BG,
+        "axes.edgecolor": INK_SOFT,
+        "axes.labelcolor": INK,
+        "text.color": INK,
+        "xtick.color": INK_SOFT,
+        "ytick.color": INK_SOFT,
+        "grid.color": INK,
+        "grid.alpha": 0.10,
+        "legend.facecolor": ELEVATED_BG,
+        "legend.edgecolor": INK_SOFT,
+    },
+)
+
+# Data - Monthly sales revenue with recession period and target threshold
 np.random.seed(42)
 months = pd.date_range(start="2006-01", periods=60, freq="ME")
 base_trend = np.linspace(100, 150, 60)
-# Dip during recession period (2008-2009)
 recession_effect = np.where((months >= "2008-01") & (months <= "2009-12"), -30 * np.sin(np.linspace(0, np.pi, 60)), 0)
 sales = base_trend + recession_effect + np.random.randn(60) * 8
-
 df = pd.DataFrame({"Month": months, "Sales": sales})
 
 # Plot
-fig, ax = plt.subplots(figsize=(16, 9))
+fig, ax = plt.subplots(figsize=(16, 9), facecolor=PAGE_BG)
 
-# Vertical span - recession period (2008-2009)
+# Vertical span — recession period (2008–2009)
 ax.axvspan(
     pd.Timestamp("2008-01-01"),
     pd.Timestamp("2009-12-31"),
     alpha=0.25,
-    color="#306998",
-    label="Recession Period (2008-2009)",
+    color=SPAN_RECESSION,
+    label="Recession (2008–2009)",
 )
 
-# Horizontal span - target sales zone (120-140)
-ax.axhspan(120, 140, alpha=0.2, color="#FFD43B", label="Target Zone (120-140)")
+# Horizontal span — target sales zone (120–140)
+ax.axhspan(120, 140, alpha=0.20, color=SPAN_TARGET, label="Target Zone (120–140)")
 
-# Line plot using seaborn
-sns.lineplot(data=df, x="Month", y="Sales", ax=ax, linewidth=3, color="#306998")
+# Line plot
+sns.lineplot(data=df, x="Month", y="Sales", ax=ax, linewidth=3, color=BRAND)
 
-# Styling
-ax.set_title("span-basic · seaborn · pyplots.ai", fontsize=24)
-ax.set_xlabel("Month", fontsize=20)
-ax.set_ylabel("Sales (thousands $)", fontsize=20)
-ax.tick_params(axis="both", labelsize=16)
+# Style
+ax.set_title("span-basic · seaborn · anyplot.ai", fontsize=24, fontweight="medium", color=INK)
+ax.set_xlabel("Month", fontsize=20, color=INK)
+ax.set_ylabel("Sales (thousands $)", fontsize=20, color=INK)
+ax.tick_params(axis="both", labelsize=16, colors=INK_SOFT)
+ax.spines["top"].set_visible(False)
+ax.spines["right"].set_visible(False)
+ax.spines["left"].set_color(INK_SOFT)
+ax.spines["bottom"].set_color(INK_SOFT)
+ax.yaxis.grid(True, alpha=0.10, linewidth=0.8, color=INK)
 ax.legend(fontsize=16, loc="upper left")
-ax.grid(True, alpha=0.3, linestyle="--")
 
 plt.tight_layout()
-plt.savefig("plot.png", dpi=300, bbox_inches="tight")
+plt.savefig(f"plot-{THEME}.png", dpi=300, bbox_inches="tight", facecolor=PAGE_BG)