feat(altair): implement line-stepwise (#2632)

## Implementation: `line-stepwise` - altair

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594559674)*

---------

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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/altair.py

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+""" pyplots.ai
+line-stepwise: Step Line Plot
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Daily temperature readings that change discretely
+np.random.seed(42)
+days = np.arange(0, 20)
+# Temperature values that step discretely (simulating daily high temperature readings)
+base_temp = 22
+temps = base_temp + np.cumsum(np.random.choice([-2, -1, 0, 1, 2], size=20))
+
+df = pd.DataFrame({"Day": days, "Temperature (°C)": temps})
+
+# Create step line chart using interpolate='step-after'
+chart = (
+    alt.Chart(df)
+    .mark_line(interpolate="step-after", strokeWidth=4, color="#306998")
+    .encode(
+        x=alt.X("Day:Q", title="Day", axis=alt.Axis(labelFontSize=18, titleFontSize=22)),
+        y=alt.Y(
+            "Temperature (°C):Q",
+            title="Temperature (°C)",
+            scale=alt.Scale(zero=False),
+            axis=alt.Axis(labelFontSize=18, titleFontSize=22),
+        ),
+        tooltip=["Day", "Temperature (°C)"],
+    )
+    .properties(
+        width=1600, height=900, title=alt.Title("line-stepwise · altair · pyplots.ai", fontSize=28, anchor="middle")
+    )
+    .configure_axis(grid=True, gridOpacity=0.3, gridDash=[4, 4])
+    .configure_view(strokeWidth=0)
+)
+
+# Save as PNG (1600 × 900 * 3 = 4800 × 2700 px)
+chart.save("plot.png", scale_factor=3.0)
+
+# Save as HTML for interactivity
+chart.save("plot.html")

plots/line-stepwise/metadata/altair.yaml

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+library: altair
+specification_id: line-stepwise
+created: '2025-12-30T10:39:38Z'
+updated: '2025-12-30T10:50:06Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594559674
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/altair/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent use of Altair declarative grammar with proper encoding types (:Q for
+    quantitative)
+  - Clean implementation of step-after interpolation which clearly demonstrates the
+    stepwise behavior
+  - 'Good use of Altair-specific features: tooltips, configure_axis for grid styling,
+    Title object with fontSize'
+  - Proper scale configuration with zero=False to focus on the data range
+  - Code is clean, well-organized, and follows KISS principles
+  weaknesses:
+  - Data shows predominantly upward trend; could benefit from more varied step patterns
+    (both increases and decreases) to better demonstrate discrete state changes