feat(letsplot): implement gantt-basic (#2405)

## Implementation: `gantt-basic` - letsplot

Implements the **letsplot** version of `gantt-basic`.

**File:** `plots/gantt-basic/implementations/letsplot.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20543284331)*

---------

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

Author: github-actions[bot]

plots/gantt-basic/implementations/letsplot.py

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+""" pyplots.ai
+gantt-basic: Basic Gantt Chart
+Library: letsplot 4.8.2 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-27
+"""
+
+import os
+from datetime import datetime
+
+import pandas as pd
+from lets_plot import *
+
+
+LetsPlot.setup_html()
+
+# Data - Software Development Project
+tasks_data = {
+    "task": [
+        "Requirements Gathering",
+        "System Design",
+        "UI/UX Design",
+        "Backend Development",
+        "Frontend Development",
+        "Database Setup",
+        "API Integration",
+        "Unit Testing",
+        "Integration Testing",
+        "User Acceptance Testing",
+        "Documentation",
+        "Deployment",
+    ],
+    "category": [
+        "Planning",
+        "Planning",
+        "Design",
+        "Development",
+        "Development",
+        "Development",
+        "Development",
+        "Testing",
+        "Testing",
+        "Testing",
+        "Documentation",
+        "Deployment",
+    ],
+    "start": [
+        datetime(2025, 1, 6),
+        datetime(2025, 1, 13),
+        datetime(2025, 1, 20),
+        datetime(2025, 1, 27),
+        datetime(2025, 2, 3),
+        datetime(2025, 1, 27),
+        datetime(2025, 2, 17),
+        datetime(2025, 2, 10),
+        datetime(2025, 2, 24),
+        datetime(2025, 3, 3),
+        datetime(2025, 2, 17),
+        datetime(2025, 3, 10),
+    ],
+    "end": [
+        datetime(2025, 1, 10),
+        datetime(2025, 1, 17),
+        datetime(2025, 1, 31),
+        datetime(2025, 2, 14),
+        datetime(2025, 2, 21),
+        datetime(2025, 2, 7),
+        datetime(2025, 2, 28),
+        datetime(2025, 2, 21),
+        datetime(2025, 3, 7),
+        datetime(2025, 3, 14),
+        datetime(2025, 3, 7),
+        datetime(2025, 3, 14),
+    ],
+}
+
+df = pd.DataFrame(tasks_data)
+
+# Convert dates to numeric for plotting (days since project start)
+project_start = datetime(2025, 1, 6)
+df["start_days"] = [(d - project_start).days for d in df["start"]]
+df["end_days"] = [(d - project_start).days for d in df["end"]]
+df["duration"] = df["end_days"] - df["start_days"]
+
+# Order tasks by start date and assign y positions
+df = df.sort_values("start_days", ascending=False).reset_index(drop=True)
+df["y_pos"] = range(len(df))
+
+# Create the Gantt chart using horizontal bars (geom_segment)
+plot = (
+    ggplot(df, aes(x="start_days", y="y_pos", color="category"))
+    + geom_segment(aes(xend="end_days", yend="y_pos"), size=12, alpha=0.85)
+    + geom_point(size=4, shape=15)  # Start markers
+    + geom_point(aes(x="end_days"), size=4, shape=15)  # End markers
+    + scale_y_continuous(breaks=list(df["y_pos"]), labels=list(df["task"]), expand=[0.05, 0.05])
+    + scale_x_continuous(
+        name="Project Timeline (Days from Start)",
+        breaks=[0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70],
+        labels=[
+            "Week 1",
+            "Week 2",
+            "Week 3",
+            "Week 4",
+            "Week 5",
+            "Week 6",
+            "Week 7",
+            "Week 8",
+            "Week 9",
+            "Week 10",
+            "Week 11",
+        ],
+    )
+    + scale_color_manual(values=["#306998", "#FFD43B", "#22C55E", "#EF4444", "#8B5CF6", "#06B6D4"], name="Phase")
+    + labs(title="gantt-basic · letsplot · pyplots.ai", x="Project Timeline", y="Tasks")
+    + theme_minimal()
+    + theme(
+        plot_title=element_text(size=28, face="bold"),
+        axis_title_x=element_text(size=22),
+        axis_title_y=element_text(size=22),
+        axis_text_x=element_text(size=16, angle=45),
+        axis_text_y=element_text(size=16),
+        legend_title=element_text(size=18),
+        legend_text=element_text(size=16),
+        legend_position="right",
+        panel_grid_major_x=element_line(color="#E5E5E5", size=0.5),
+        panel_grid_major_y=element_line(color="#F5F5F5", size=0.3),
+        panel_grid_minor=element_blank(),
+    )
+    + ggsize(1600, 900)
+)
+
+# Save as PNG (scale 3x for 4800 × 2700 px)
+ggsave(plot, "plot.png", scale=3, path=".")
+
+# Save interactive HTML version
+ggsave(plot, "plot.html", path=".")
+
+# Clean up lets-plot-images folder if created
+if os.path.exists("lets-plot-images"):
+    import shutil
+
+    shutil.rmtree("lets-plot-images")

plots/gantt-basic/metadata/letsplot.yaml

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+library: letsplot
+specification_id: gantt-basic
+created: '2025-12-27T19:22:28Z'
+updated: '2025-12-27T19:25:45Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20543284331
+issue: 0
+python_version: 3.13.11
+library_version: 4.8.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/letsplot/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/letsplot/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/letsplot/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent visual clarity with proper bar sizing and spacing between tasks
+  - Smart use of geom_segment with square end markers for clean Gantt bar appearance
+  - Professional color palette with 6 distinct phase colors
+  - Well-structured realistic software development project data
+  - Proper canvas sizing (1600x900 with scale=3) for 4800x2700 output
+  - Clean week-based x-axis labeling making timeline easy to follow
+  - Tasks logically sorted by start date for natural reading flow
+  weaknesses:
+  - Missing today marker (vertical reference line) which spec notes suggest
+  - Blue and teal colors for Planning and Deployment phases could be more distinct
+    for colorblind accessibility
+  - Cleanup code for lets-plot-images folder adds unnecessary complexity