feat(pygal): implement line-stepwise (#2638)

## Implementation: `line-stepwise` - pygal

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594562908)*

---------

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

Author: github-actions[bot]

plots/line-stepwise/implementations/pygal.py

@@ -0,0 +1,91 @@
+""" pyplots.ai
+line-stepwise: Step Line Plot
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import pygal
+from pygal.style import Style
+
+
+# Data - Server response time stepping over 24 hours
+hours = list(range(0, 25))  # 0-24 hours
+
+# Response time that steps discretely (server load changes)
+response_times = [50]  # Start at 50ms
+for i in range(1, 25):
+    # Discrete jumps at certain hours (morning rush, lunch, evening)
+    if i == 8:  # Morning rush
+        response_times.append(120)
+    elif i == 12:  # Lunch peak
+        response_times.append(180)
+    elif i == 14:  # Post lunch
+        response_times.append(100)
+    elif i == 18:  # Evening rush
+        response_times.append(200)
+    elif i == 21:  # Night decrease
+        response_times.append(80)
+    elif i == 23:  # Late night
+        response_times.append(40)
+    else:
+        response_times.append(response_times[-1])  # Stay constant
+
+# Create step data by duplicating points for horizontal-then-vertical transitions
+# For step-post style: horizontal segment at old value, then vertical jump
+step_x_labels = []
+step_values = []
+for i, (h, v) in enumerate(zip(hours, response_times, strict=True)):
+    if i == 0:
+        step_x_labels.append(str(h))
+        step_values.append(v)
+    else:
+        # Add point at new x with OLD y value (horizontal segment)
+        step_x_labels.append(str(h))
+        step_values.append(response_times[i - 1])
+        # Add point at same x with NEW y value (vertical segment)
+        step_x_labels.append("")  # Empty label to avoid clutter
+        step_values.append(v)
+
+# Custom style for pyplots
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998",),  # Python Blue
+    title_font_size=56,
+    label_font_size=36,
+    major_label_font_size=32,
+    legend_font_size=32,
+    value_font_size=28,
+    stroke_width=5,
+)
+
+# Create line chart
+chart = pygal.Line(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="line-stepwise · pygal · pyplots.ai",
+    x_title="Hour of Day",
+    y_title="Response Time (ms)",
+    show_dots=False,  # Hide dots for cleaner step appearance
+    fill=False,
+    show_x_guides=False,
+    show_y_guides=True,
+    show_legend=False,
+    truncate_label=-1,
+    show_minor_x_labels=True,
+    x_label_rotation=0,
+)
+
+# X-axis labels
+chart.x_labels = step_x_labels
+
+# Add step data
+chart.add("Server Response", step_values)
+
+# Save outputs
+chart.render_to_png("plot.png")
+chart.render_to_file("plot.html")

plots/line-stepwise/metadata/pygal.yaml

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+library: pygal
+specification_id: line-stepwise
+created: '2025-12-30T10:42:35Z'
+updated: '2025-12-30T10:50:51Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594562908
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent step visualization achieved through clever data point duplication for
+    horizontal-then-vertical transitions
+  - Clean, professional appearance with appropriate Python Blue color scheme
+  - Realistic server response time scenario that clearly demonstrates discrete state
+    changes
+  - Proper title format and descriptive axis labels with units
+  - Good font sizing for the 4800x2700 canvas
+  weaknesses:
+  - Manual step data construction is verbose; pygal native line interpolation options
+    could be explored
+  - Right margin has slight excess whitespace
+  - Grid only shows Y-axis guides; X-axis guides could improve readability of hour
+    markers