feat(pygal): implement bar-error (#2402)

## Implementation: `bar-error` - pygal

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20543283514)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/bar-error/implementations/pygal.py

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+""" pyplots.ai
+bar-error: Bar Chart with Error Bars
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-27
+"""
+
+import pygal
+from pygal.style import Style
+
+
+# Data: Experimental results comparing treatment effectiveness
+# Mean values with standard deviations (±1 SD)
+categories = ["Control", "Treatment A", "Treatment B", "Treatment C", "Treatment D"]
+values = [45.2, 62.8, 78.3, 55.1, 71.5]
+errors = [8.5, 12.3, 9.7, 15.2, 11.8]
+
+# Custom style for 4800x2700 canvas
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#4CAF50", "#E91E63", "#9C27B0"),
+    font_family="DejaVu Sans, Verdana, sans-serif",
+    title_font_size=72,
+    label_font_size=48,
+    major_label_font_size=48,
+    legend_font_size=48,
+    value_font_size=36,
+    value_label_font_size=36,
+    tooltip_font_size=36,
+)
+
+# Create bar chart with error bars (confidence intervals)
+chart = pygal.Bar(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="bar-error \u00b7 pygal \u00b7 pyplots.ai",
+    x_title="Treatment Group",
+    y_title="Response Value (units)",
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_at_bottom_columns=1,
+    show_y_guides=True,
+    show_x_guides=False,
+    print_values=False,
+    range=(0, 100),
+    spacing=40,
+    margin=100,
+    margin_bottom=250,
+    margin_left=200,
+    margin_top=180,
+    dots_size=8,
+    stroke_style={"width": 4},
+)
+
+# X-axis labels
+chart.x_labels = categories
+
+# Add data with confidence intervals (error bars)
+# Each value is a dict with 'value' and 'ci' containing 'low' and 'high'
+data_with_errors = []
+for val, err in zip(values, errors, strict=True):
+    data_with_errors.append({"value": val, "ci": {"low": val - err, "high": val + err}})
+
+chart.add("Mean \u00b1 1 SD", data_with_errors)
+
+# Save as PNG and HTML (interactive)
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/bar-error/metadata/pygal.yaml

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+library: pygal
+specification_id: bar-error
+created: '2025-12-27T19:22:12Z'
+updated: '2025-12-27T19:29:48Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20543283514
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/bar-error/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/bar-error/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/bar-error/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent use of pygal native confidence interval (ci) feature for error bars
+    with caps
+  - Clean professional appearance with appropriate font scaling for 4800x2700 canvas
+  - Well-chosen scientific context (treatment comparison) that matches spec applications
+  - Proper title format and comprehensive axis labeling with units
+  - Good use of custom Style for consistent visual appearance
+  weaknesses:
+  - Legend placement at bottom-left corner appears disconnected from the chart
+  - Grid lines could be more subtle (currently dotted but still prominent)
+  - Only symmetric error bars shown when spec mentions asymmetric as an option