feat(letsplot): implement line-animated-progressive (#3277)

## Implementation: `line-animated-progressive` - letsplot

Implements the **letsplot** version of `line-animated-progressive`.

**File:** `plots/line-animated-progressive/implementations/letsplot.py`

**Parent Issue:** #3238

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20795233520)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: github-actions[bot]

plots/line-animated-progressive/implementations/letsplot.py

@@ -0,0 +1,87 @@
+"""pyplots.ai
+line-animated-progressive: Animated Line Plot Over Time
+Library: lets-plot | Python 3.13
+Quality: pending | Created: 2025-01-07
+"""
+
+import numpy as np
+import pandas as pd
+from lets_plot import (
+    LetsPlot,
+    aes,
+    element_line,
+    element_rect,
+    element_text,
+    facet_wrap,
+    geom_line,
+    geom_point,
+    ggplot,
+    ggsize,
+    labs,
+    scale_color_manual,
+    scale_x_continuous,
+    theme,
+    theme_minimal,
+)
+from lets_plot.export import ggsave
+
+
+LetsPlot.setup_html()
+
+# Data - Monthly website traffic over 2 years
+np.random.seed(42)
+n_points = 24
+months = np.arange(1, n_points + 1)
+
+# Traffic with seasonal pattern and growth trend
+base_traffic = 50000
+trend = np.linspace(0, 30000, n_points)
+seasonal = 8000 * np.sin(2 * np.pi * months / 12)
+noise = np.random.normal(0, 3000, n_points)
+traffic = base_traffic + trend + seasonal + noise
+
+# Create small multiples showing progressive stages
+stages = [6, 12, 18, 24]
+stage_labels = ["1. Q2 2023", "2. Q4 2023", "3. Q2 2024", "4. Complete"]
+
+dfs = []
+for stage, label in zip(stages, stage_labels, strict=True):
+    stage_df = pd.DataFrame(
+        {
+            "month": months[:stage],
+            "traffic": traffic[:stage],
+            "stage": label,
+            "is_latest": [False] * (stage - 1) + [True],
+        }
+    )
+    dfs.append(stage_df)
+
+df = pd.concat(dfs, ignore_index=True)
+
+# Create plot with small multiples
+plot = (
+    ggplot(df, aes(x="month", y="traffic"))
+    + geom_line(color="#306998", size=2, alpha=0.9)
+    + geom_point(aes(color="is_latest"), size=4, alpha=0.8)
+    + scale_color_manual(values={"True": "#FFD43B", "False": "#306998"}, guide="none")
+    + facet_wrap("stage", ncol=2)
+    + scale_x_continuous(breaks=[6, 12, 18, 24], labels=["Jun '23", "Dec '23", "Jun '24", "Dec '24"])
+    + labs(title="line-animated-progressive \u00b7 letsplot \u00b7 pyplots.ai", x="Time Period", y="Monthly Visitors")
+    + theme_minimal()
+    + theme(
+        plot_title=element_text(size=26),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        strip_text=element_text(size=18),
+        legend_text=element_text(size=16),
+        panel_grid=element_line(color="#CCCCCC", size=0.4, linetype="dashed"),
+        panel_background=element_rect(fill="#FAFAFA"),
+    )
+    + ggsize(1600, 900)
+)
+
+# Save PNG (scale=3 gives 4800x2700)
+ggsave(plot, "plot.png", path=".", scale=3)
+
+# Save HTML for interactivity
+ggsave(plot, "plot.html", path=".")

plots/line-animated-progressive/metadata/letsplot.yaml

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+# Per-library metadata for letsplot implementation of line-animated-progressive
+# Auto-generated by impl-generate.yml
+
+library: letsplot
+specification_id: line-animated-progressive
+created: '2026-01-07T20:31:31Z'
+updated: '2026-01-07T20:31:31Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20795233520
+issue: 3238
+python_version: 3.13.11
+library_version: 4.8.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-animated-progressive/letsplot/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-animated-progressive/letsplot/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-animated-progressive/letsplot/plot.html
+quality_score: null
+review:
+  strengths: []
+  weaknesses: []