feat(pygal): implement histogram-stepwise (#2657)

## Implementation: `histogram-stepwise` - pygal

Implements the **pygal** version of `histogram-stepwise`.

**File:** `plots/histogram-stepwise/implementations/pygal.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20595338040)*

---------

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

Author: github-actions[bot]

plots/histogram-stepwise/implementations/pygal.py

@@ -0,0 +1,74 @@
+""" pyplots.ai
+histogram-stepwise: Step Histogram
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pygal
+from pygal.style import Style
+
+
+# Data
+np.random.seed(42)
+values = np.concatenate([np.random.normal(50, 10, 200), np.random.normal(80, 8, 150)])
+
+# Compute histogram bins
+counts, bin_edges = np.histogram(values, bins=25)
+
+# Build step coordinates for pygal.XY
+# Step histogram: horizontal segments at count levels, vertical connections
+step_data = []
+for i in range(len(counts)):
+    left = bin_edges[i]
+    right = bin_edges[i + 1]
+    count = counts[i]
+    # Horizontal segment for this bin
+    step_data.append((left, count))
+    step_data.append((right, count))
+
+# Add baseline at start and end
+step_data.insert(0, (bin_edges[0], 0))
+step_data.append((bin_edges[-1], 0))
+
+# Custom style for large canvas
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998",),
+    title_font_size=48,
+    label_font_size=36,
+    major_label_font_size=32,
+    legend_font_size=32,
+    value_font_size=28,
+    stroke_width=4,
+    opacity=1.0,
+    opacity_hover=1.0,
+)
+
+# Create XY chart for step lines
+chart = pygal.XY(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="histogram-stepwise · pygal · pyplots.ai",
+    x_title="Value",
+    y_title="Frequency",
+    show_dots=False,
+    stroke_style={"width": 4},
+    fill=False,
+    show_legend=False,
+    show_y_guides=True,
+    show_x_guides=False,
+    x_label_rotation=0,
+)
+
+# Add step line data
+chart.add("Distribution", step_data)
+
+# Save outputs
+chart.render_to_file("plot.html")
+chart.render_to_png("plot.png")

plots/histogram-stepwise/metadata/pygal.yaml

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+library: pygal
+specification_id: histogram-stepwise
+created: '2025-12-30T11:22:08Z'
+updated: '2025-12-30T11:29:13Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20595338040
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/histogram-stepwise/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/histogram-stepwise/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/histogram-stepwise/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent implementation of step histogram using XY chart with carefully constructed
+    step coordinates
+  - Clean, readable code with proper KISS structure and reproducible seed
+  - Bimodal distribution data effectively showcases the plot type ability to reveal
+    distribution shapes
+  - Proper title format and clear axis labels
+  - Good font sizing for the 4800x2700 canvas
+  weaknesses:
+  - Axis labels lack units (e.g., Value (units) or Frequency (count))
+  - Could use more realistic/contextual data scenario instead of abstract values