feat(plotnine): implement line-stepwise (#2609)

## Implementation: `line-stepwise` - plotnine

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594559603)*

---------

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

Author: github-actions[bot]

plots/line-stepwise/implementations/plotnine.py

@@ -0,0 +1,65 @@
+""" pyplots.ai
+line-stepwise: Step Line Plot
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import aes, element_text, geom_step, ggplot, labs, theme, theme_minimal
+
+
+# Data - Server CPU utilization showing discrete state changes
+np.random.seed(42)
+hours = np.arange(0, 24)
+# Simulate CPU utilization that changes in discrete steps
+base_utilization = np.array(
+    [
+        15,
+        15,
+        12,
+        10,
+        10,
+        20,  # Night/early morning - low usage
+        45,
+        65,
+        75,
+        80,
+        85,
+        80,  # Morning ramp-up - high load
+        70,
+        75,
+        80,
+        85,
+        90,
+        85,  # Afternoon - peak hours
+        70,
+        55,
+        40,
+        30,
+        25,
+        18,  # Evening wind-down
+    ]
+)
+
+df = pd.DataFrame({"hour": hours, "cpu_utilization": base_utilization})
+
+# Plot
+plot = (
+    ggplot(df, aes(x="hour", y="cpu_utilization"))
+    + geom_step(color="#306998", size=2, direction="hv")
+    + labs(x="Hour of Day", y="CPU Utilization (%)", title="line-stepwise · plotnine · pyplots.ai")
+    + theme_minimal()
+    + theme(
+        figure_size=(16, 9),
+        text=element_text(size=14),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        plot_title=element_text(size=24),
+        panel_grid_major=element_text(color="#cccccc"),
+        panel_grid_minor=element_text(color="#eeeeee"),
+    )
+)
+
+# Save
+plot.save("plot.png", dpi=300, verbose=False)

plots/line-stepwise/metadata/plotnine.yaml

@@ -0,0 +1,25 @@
+library: plotnine
+specification_id: line-stepwise
+created: '2025-12-30T10:37:30Z'
+updated: '2025-12-30T10:43:58Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594559603
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 92
+review:
+  strengths:
+  - Excellent use of geom_step with direction="hv" for proper step function visualization
+  - Realistic and relatable CPU utilization scenario that perfectly demonstrates discrete
+    state changes
+  - Clean grammar of graphics implementation following plotnine best practices
+  - Well-balanced layout with appropriate text sizing for 4800x2700 output
+  - Data shows clear daily pattern with morning ramp-up, business hours peak, and
+    evening wind-down
+  weaknesses:
+  - 'Minor: panel_grid_major and panel_grid_minor use element_text() instead of element_line()
+    - this does not cause visible issues but is technically incorrect for grid styling'