feat(plotnine): implement line-markers (#2608)

## Implementation: `line-markers` - plotnine

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594558022)*

---------

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

Author: github-actions[bot]

plots/line-markers/implementations/plotnine.py

@@ -0,0 +1,70 @@
+""" pyplots.ai
+line-markers: Line Plot with Markers
+Library: plotnine 0.15.2 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from plotnine import (
+    aes,
+    element_text,
+    geom_line,
+    geom_point,
+    ggplot,
+    labs,
+    scale_color_manual,
+    scale_shape_manual,
+    theme,
+    theme_minimal,
+)
+
+
+# Data: Monthly temperature readings from two weather stations
+np.random.seed(42)
+months = np.arange(1, 13)
+month_labels = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
+
+# Station A: Coastal location (milder temperatures)
+station_a = np.array([8, 9, 12, 15, 19, 23, 26, 26, 22, 17, 12, 9]) + np.random.randn(12) * 0.5
+
+# Station B: Inland location (more extreme temperatures)
+station_b = np.array([2, 4, 10, 16, 21, 26, 29, 28, 22, 14, 7, 3]) + np.random.randn(12) * 0.5
+
+df = pd.DataFrame(
+    {
+        "Month": month_labels * 2,
+        "Month_Num": list(months) * 2,
+        "Temperature": np.concatenate([station_a, station_b]),
+        "Station": ["Coastal Station"] * 12 + ["Inland Station"] * 12,
+    }
+)
+
+# Plot
+plot = (
+    ggplot(df, aes(x="Month_Num", y="Temperature", color="Station", shape="Station"))
+    + geom_line(size=1.5, alpha=0.8)
+    + geom_point(size=5, alpha=0.9)
+    + scale_color_manual(values=["#306998", "#FFD43B"])
+    + scale_shape_manual(values=["o", "s"])
+    + labs(
+        x="Month",
+        y="Temperature (°C)",
+        title="line-markers · plotnine · pyplots.ai",
+        color="Weather Station",
+        shape="Weather Station",
+    )
+    + 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),
+        legend_text=element_text(size=16),
+        legend_title=element_text(size=18),
+        legend_position="right",
+    )
+)
+
+plot.save("plot.png", dpi=300, verbose=False)

plots/line-markers/metadata/plotnine.yaml

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+library: plotnine
+specification_id: line-markers
+created: '2025-12-30T10:37:30Z'
+updated: '2025-12-30T10:43:01Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594558022
+issue: 0
+python_version: 3.13.11
+library_version: 0.15.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-markers/plotnine/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-markers/plotnine/plot_thumb.png
+preview_html: null
+quality_score: 91
+review:
+  strengths:
+  - Excellent use of plotnine grammar of graphics with proper aesthetic mappings for
+    color and shape
+  - 'Well-chosen colorblind-safe color palette (blue #306998 and yellow #FFD43B)'
+  - Realistic and meaningful data scenario comparing coastal vs inland temperature
+    patterns
+  - Proper marker and line sizing for visibility at the target resolution
+  - Clean readable code following KISS principles
+  weaknesses:
+  - X-axis displays numeric month values (1, 2.5, 5, 7.5, 10, 12.5) instead of month
+    names (Jan, Feb, etc.) - the month_labels were created but not used on the axis