feat(plotnine): implement histogram-stacked (#2628)

## Implementation: `histogram-stacked` - plotnine

Implements the **plotnine** version of `histogram-stacked`.

**File:** `plots/histogram-stacked/implementations/plotnine.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594560694)*

---------

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/histogram-stacked/implementations/plotnine.py

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+""" pyplots.ai
+histogram-stacked: Stacked Histogram
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 97/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import aes, element_text, geom_histogram, ggplot, labs, scale_fill_manual, theme, theme_minimal
+
+
+# Data - Plant growth measurements across different soil types
+np.random.seed(42)
+
+# Generate realistic plant height data for three soil types
+soil_a = np.random.normal(loc=25, scale=5, size=150)  # Sandy soil
+soil_b = np.random.normal(loc=30, scale=6, size=120)  # Loamy soil
+soil_c = np.random.normal(loc=22, scale=4, size=100)  # Clay soil
+
+df = pd.DataFrame(
+    {
+        "height": np.concatenate([soil_a, soil_b, soil_c]),
+        "soil_type": (["Sandy Soil"] * 150 + ["Loamy Soil"] * 120 + ["Clay Soil"] * 100),
+    }
+)
+
+# Create stacked histogram
+plot = (
+    ggplot(df, aes(x="height", fill="soil_type"))
+    + geom_histogram(bins=20, position="stack", alpha=0.85, color="white", size=0.3)
+    + labs(x="Plant Height (cm)", y="Frequency", title="histogram-stacked · plotnine · pyplots.ai", fill="Soil Type")
+    + scale_fill_manual(values=["#306998", "#FFD43B", "#4ECDC4"])
+    + theme_minimal()
+    + theme(
+        figure_size=(16, 9),
+        text=element_text(size=14),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        plot_title=element_text(size=24),
+        legend_text=element_text(size=16),
+        legend_title=element_text(size=18),
+        legend_position="right",
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300)

plots/histogram-stacked/metadata/plotnine.yaml

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+library: plotnine
+specification_id: histogram-stacked
+created: '2025-12-30T10:39:17Z'
+updated: '2025-12-30T10:49:29Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594560694
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/histogram-stacked/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/histogram-stacked/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 97
+review:
+  strengths:
+  - Excellent use of plotnine grammar of graphics with proper stacking via position=stack
+  - Comprehensive theme customization with appropriate font sizes for 4800x2700 output
+  - Realistic and engaging data scenario (plant growth across soil types)
+  - Perfect title format and axis labels with units
+  - Clean KISS code structure with reproducible seed
+  - White bar borders enhance visual separation between stacked segments
+  weaknesses:
+  - Could use scale_fill_brewer for more colorblind-safe colors instead of manual
+    hex values