feat(matplotlib): implement bar-error (#2391)

## Implementation: `bar-error` - matplotlib

Implements the **matplotlib** version of `bar-error`.

**File:** `plots/bar-error/implementations/matplotlib.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20543280641)*

---------

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

Author: github-actions[bot]

plots/bar-error/implementations/matplotlib.py

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+""" pyplots.ai
+bar-error: Bar Chart with Error Bars
+Library: matplotlib 3.10.8 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-27
+"""
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+
+# Data: A/B test results comparing feature variants with confidence intervals
+np.random.seed(42)
+categories = ["Control", "Variant A", "Variant B", "Variant C", "Variant D"]
+values = [12.3, 15.8, 14.2, 18.5, 11.7]  # Conversion rates (%)
+errors = [1.2, 2.1, 1.5, 2.8, 1.0]  # 95% CI half-widths
+
+# Create figure
+fig, ax = plt.subplots(figsize=(16, 9))
+
+# Bar positions
+x = np.arange(len(categories))
+bar_width = 0.6
+
+# Plot bars with error bars
+bars = ax.bar(x, values, bar_width, color="#306998", edgecolor="#1e4466", linewidth=2)
+ax.errorbar(x, values, yerr=errors, fmt="none", ecolor="#1e4466", elinewidth=3, capsize=10, capthick=3)
+
+# Labels and styling
+ax.set_xlabel("Test Group", fontsize=20)
+ax.set_ylabel("Conversion Rate (%)", fontsize=20)
+ax.set_title("bar-error · matplotlib · pyplots.ai", fontsize=24)
+ax.set_xticks(x)
+ax.set_xticklabels(categories, fontsize=16)
+ax.tick_params(axis="y", labelsize=16)
+
+# Add annotation explaining error bars
+ax.annotate(
+    "Error bars: 95% CI",
+    xy=(0.98, 0.02),
+    xycoords="axes fraction",
+    fontsize=14,
+    ha="right",
+    va="bottom",
+    bbox={"boxstyle": "round,pad=0.3", "facecolor": "white", "edgecolor": "gray", "alpha": 0.8},
+)
+
+# Grid and layout
+ax.grid(True, axis="y", alpha=0.3, linestyle="--")
+ax.set_ylim(0, max(values) * 1.3)
+
+plt.tight_layout()
+plt.savefig("plot.png", dpi=300, bbox_inches="tight")

plots/bar-error/metadata/matplotlib.yaml

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+library: matplotlib
+specification_id: bar-error
+created: '2025-12-27T19:21:04Z'
+updated: '2025-12-27T19:26:22Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20543280641
+issue: 0
+python_version: 3.13.11
+library_version: 3.10.8
+preview_url: https://storage.googleapis.com/pyplots-images/plots/bar-error/matplotlib/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/bar-error/matplotlib/plot_thumb.png
+preview_html: null
+quality_score: 92
+review:
+  strengths:
+  - Excellent text legibility with appropriate font sizes for all elements
+  - Clear, realistic A/B test scenario with plausible conversion rate data
+  - Error bar caps are clearly visible with good sizing (capsize=10, capthick=3)
+  - Clean annotation explaining what error bars represent (95% CI requirement from
+    spec)
+  - Good color choice with consistent dark blue theme
+  - KISS code structure following matplotlib best practices
+  weaknesses:
+  - Error bars are all symmetric; demonstrating asymmetric errors would better showcase
+    the plot type capabilities
+  - Annotation box placement in bottom-right corner floats in empty space rather than
+    being more integrated with the plot
+  - Basic library usage without leveraging advanced matplotlib features like bar_label()
+    for showing values