feat(matplotlib): implement span-basic (#5610)

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

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

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

**Parent Issue:** #980

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25160721973)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

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

@@ -1,45 +1,66 @@
-""" pyplots.ai
+""" anyplot.ai
 span-basic: Basic Span Plot (Highlighted Region)
-Library: matplotlib 3.10.8 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-23
+Library: matplotlib 3.10.9 | Python 3.13.13
+Quality: 91/100 | Updated: 2026-04-30
 """
 
+import os
+
 import matplotlib.pyplot as plt
 import numpy as np
 
 
-# Data - Stock prices with highlighted recession period
-np.random.seed(42)
-dates = np.arange(2006, 2016, 0.1)  # 10 years of data
+# 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"
+INK_MUTED = "#6B6A63" if THEME == "light" else "#A8A79F"
 
-# Simulate stock price with trend and volatility
-price = 100 + np.cumsum(np.random.randn(len(dates)) * 2)
+BRAND = "#009E73"  # Okabe-Ito position 1 — first series
+C2 = "#D55E00"  # Okabe-Ito position 2
+C3 = "#0072B2"  # Okabe-Ito position 3
+
+# Data — stock prices with a simulated recession dip
+np.random.seed(42)
+dates = np.arange(2004, 2016, 0.1)
 
-# Add a dip during recession period (2008-2009)
+price = 100 + np.cumsum(np.random.randn(len(dates)) * 1.5)
 recession_mask = (dates >= 2008) & (dates < 2010)
-price[recession_mask] -= np.linspace(0, 30, recession_mask.sum())
-price[dates >= 2010] -= 30
-price = price + np.abs(price.min()) + 50  # Keep positive
+price[recession_mask] -= np.linspace(0, 35, recession_mask.sum())
+price[dates >= 2010] -= 35
+price = price - price.min() + 70
+
+# Plot
+fig, ax = plt.subplots(figsize=(16, 9), facecolor=PAGE_BG)
+ax.set_facecolor(PAGE_BG)
+
+ax.plot(dates, price, linewidth=3, color=BRAND, label="Stock Price Index")
+
+# Vertical span — recession period (2008–2009)
+ax.axvspan(2008, 2010, alpha=0.22, color=C2, label="Recession Period")
 
-# Create plot (4800x2700 px)
-fig, ax = plt.subplots(figsize=(16, 9))
+# Horizontal span — risk zone (low values)
+ax.axhspan(70, 95, alpha=0.18, color=C3, label="Risk Zone")
 
-# Plot line data
-ax.plot(dates, price, linewidth=3, color="#306998", label="Stock Price")
+# Style
+ax.set_xlabel("Year", fontsize=20, color=INK)
+ax.set_ylabel("Price Index", fontsize=20, color=INK)
+ax.set_title("span-basic · matplotlib · anyplot.ai", fontsize=24, fontweight="medium", color=INK)
+ax.tick_params(axis="both", labelsize=16, colors=INK_SOFT, labelcolor=INK_SOFT)
 
-# Vertical span highlighting recession period (2008-2009)
-ax.axvspan(2008, 2009, alpha=0.25, color="#FFD43B", label="Recession Period")
+ax.spines["top"].set_visible(False)
+ax.spines["right"].set_visible(False)
+for spine in ("left", "bottom"):
+    ax.spines[spine].set_color(INK_SOFT)
 
-# Horizontal span highlighting danger zone (low values)
-ax.axhspan(60, 80, alpha=0.2, color="#D62728", label="Risk Zone")
+ax.yaxis.grid(True, alpha=0.12, linewidth=0.8, color=INK)
 
-# Labels and styling
-ax.set_xlabel("Year", fontsize=20)
-ax.set_ylabel("Price ($)", fontsize=20)
-ax.set_title("span-basic · matplotlib · pyplots.ai", fontsize=24)
-ax.tick_params(axis="both", labelsize=16)
-ax.grid(True, alpha=0.3, linestyle="--")
-ax.legend(fontsize=16, loc="upper left")
+leg = ax.legend(fontsize=16, loc="upper left")
+leg.get_frame().set_facecolor(ELEVATED_BG)
+leg.get_frame().set_edgecolor(INK_SOFT)
+plt.setp(leg.get_texts(), color=INK_SOFT)
 
 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)