feat(pygal): implement gantt-basic (#2418)

## Implementation: `gantt-basic` - pygal

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

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

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20543283930)*

---------

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/gantt-basic/implementations/pygal.py

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+""" pyplots.ai
+gantt-basic: Basic Gantt Chart
+Library: pygal 3.1.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-27
+"""
+
+from datetime import date
+
+import cairosvg
+import pygal
+from pygal.style import Style
+
+
+# Custom style for large canvas (4800x2700)
+custom_style = Style(
+    background="white",
+    plot_background="white",
+    foreground="#333333",
+    foreground_strong="#333333",
+    foreground_subtle="#666666",
+    colors=("#306998", "#FFD43B", "#2E8B57", "#DC143C", "#9370DB"),
+    title_font_size=60,
+    label_font_size=28,
+    major_label_font_size=28,
+    legend_font_size=28,
+    value_font_size=24,
+    tooltip_font_size=24,
+)
+
+# Project data: Software Development Project
+# Each tuple: (task_name, category, start_date, end_date)
+tasks = [
+    ("Requirements Analysis", "Planning", date(2025, 1, 6), date(2025, 1, 17)),
+    ("System Design", "Planning", date(2025, 1, 13), date(2025, 1, 31)),
+    ("Database Design", "Design", date(2025, 1, 27), date(2025, 2, 7)),
+    ("UI/UX Design", "Design", date(2025, 1, 20), date(2025, 2, 14)),
+    ("Backend Development", "Development", date(2025, 2, 3), date(2025, 3, 14)),
+    ("Frontend Development", "Development", date(2025, 2, 10), date(2025, 3, 21)),
+    ("API Integration", "Development", date(2025, 3, 3), date(2025, 3, 21)),
+    ("Unit Testing", "Testing", date(2025, 3, 10), date(2025, 3, 28)),
+    ("Integration Testing", "Testing", date(2025, 3, 24), date(2025, 4, 11)),
+    ("User Acceptance Testing", "Testing", date(2025, 4, 7), date(2025, 4, 18)),
+    ("Documentation", "Deployment", date(2025, 4, 7), date(2025, 4, 18)),
+    ("Deployment", "Deployment", date(2025, 4, 14), date(2025, 4, 25)),
+]
+
+# Reference date for calculations
+reference_date = date(2025, 1, 1)
+
+# Category colors matching the style
+category_colors = {
+    "Planning": "#306998",
+    "Design": "#FFD43B",
+    "Development": "#2E8B57",
+    "Testing": "#DC143C",
+    "Deployment": "#9370DB",
+}
+
+# Calculate date range
+all_dates = []
+for _, _, start, end in tasks:
+    all_dates.extend([start, end])
+min_date = min(all_dates)
+max_date = max(all_dates)
+
+# X-axis range in days
+start_day = (min_date - reference_date).days - 3
+end_day = (max_date - reference_date).days + 3
+day_range = end_day - start_day
+
+# Create task labels for y-axis
+task_labels = [t[0] for t in tasks]
+num_tasks = len(tasks)
+
+# Create pygal HorizontalBar chart as base structure
+chart = pygal.HorizontalBar(
+    width=4800,
+    height=2700,
+    style=custom_style,
+    title="Software Development Timeline · gantt-basic · pygal · pyplots.ai",
+    show_legend=True,
+    legend_at_bottom=True,
+    legend_box_size=24,
+    print_values=False,
+    show_y_guides=False,
+    show_x_guides=False,  # Hide default x guides
+    show_x_labels=False,  # Hide default 0-100 labels
+    margin=60,
+    spacing=20,
+    range=(0, 100),
+)
+
+# Set task names as y-axis labels
+chart.x_labels = task_labels
+
+# Add placeholder series for legend (one per category)
+categories_in_order = ["Planning", "Design", "Development", "Testing", "Deployment"]
+for cat in categories_in_order:
+    chart.add(cat, [None] * num_tasks)
+
+# Render base SVG
+svg_string = chart.render().decode("utf-8")
+
+# Pygal coordinates (from actual SVG: transform="translate(466, 140)", plot size 4273.6x2368)
+PLOT_ORIGIN_X = 466
+PLOT_ORIGIN_Y = 140
+PLOT_WIDTH = 4273.6
+PLOT_HEIGHT = 2368.0
+
+# Convert to absolute coordinates for injection
+PLOT_LEFT = PLOT_ORIGIN_X
+PLOT_TOP = PLOT_ORIGIN_Y
+
+row_height = PLOT_HEIGHT / num_tasks
+bar_height = row_height * 0.55
+
+# Build custom Gantt bar rectangles
+bar_elements = []
+
+for i, (task_name, category, start, end) in enumerate(tasks):
+    start_day_val = (start - reference_date).days
+    end_day_val = (end - reference_date).days
+
+    # Convert days to x position
+    x_start = PLOT_LEFT + ((start_day_val - start_day) / day_range) * PLOT_WIDTH
+    x_end = PLOT_LEFT + ((end_day_val - start_day) / day_range) * PLOT_WIDTH
+    width = x_end - x_start
+
+    # Y position (pygal HorizontalBar has first item at bottom, so reverse index)
+    reversed_i = num_tasks - 1 - i
+    y_center = PLOT_TOP + (reversed_i + 0.5) * row_height
+    y_top = y_center - bar_height / 2
+
+    color = category_colors[category]
+    duration = (end - start).days
+
+    bar_elements.append(
+        f'<rect x="{x_start:.1f}" y="{y_top:.1f}" width="{width:.1f}" '
+        f'height="{bar_height:.1f}" fill="{color}" rx="6" ry="6" opacity="0.9">'
+        f"<title>{task_name}&#10;{start.strftime('%b %d')} - {end.strftime('%b %d')} "
+        f"({duration} days)</title></rect>"
+    )
+
+# Add month markers and labels
+month_markers = []
+for month in range(1, 5):
+    month_date = date(2025, month, 1)
+    day_val = (month_date - reference_date).days
+    if start_day <= day_val <= end_day:
+        x_pos = PLOT_LEFT + ((day_val - start_day) / day_range) * PLOT_WIDTH
+        month_name = month_date.strftime("%b")
+        # Vertical guide line
+        month_markers.append(
+            f'<line x1="{x_pos:.1f}" y1="{PLOT_TOP}" x2="{x_pos:.1f}" '
+            f'y2="{PLOT_TOP + PLOT_HEIGHT}" stroke="#bbb" stroke-width="2" '
+            f'stroke-dasharray="8,4"/>'
+        )
+        # Month label
+        month_markers.append(
+            f'<text x="{x_pos:.1f}" y="{PLOT_TOP + PLOT_HEIGHT + 40}" '
+            f'font-family="Consolas, monospace" font-size="28" fill="#333" '
+            f'text-anchor="middle">{month_name} 1</text>'
+        )
+
+# X-axis title
+month_markers.append(
+    f'<text x="{PLOT_LEFT + PLOT_WIDTH / 2}" y="{PLOT_TOP + PLOT_HEIGHT + 90}" '
+    f'font-family="Consolas, monospace" font-size="32" fill="#333" '
+    f'text-anchor="middle">Timeline (2025)</text>'
+)
+
+# Inject custom elements before </svg>
+all_elements = "\n".join(bar_elements + month_markers)
+svg_output = svg_string.replace("</svg>", f"{all_elements}\n</svg>")
+
+# Remove "No data" text that appears from empty series
+svg_output = svg_output.replace(">No data<", "><")
+
+# Save outputs
+with open("plot.html", "w") as f:
+    f.write(svg_output)
+
+cairosvg.svg2png(bytestring=svg_output.encode(), write_to="plot.png")

plots/gantt-basic/metadata/pygal.yaml

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+library: pygal
+specification_id: gantt-basic
+created: '2025-12-27T19:25:30Z'
+updated: '2025-12-27T19:33:55Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20543283930
+issue: 0
+python_version: 3.13.11
+library_version: 3.1.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/pygal/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/pygal/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/gantt-basic/pygal/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent visual presentation with clear horizontal bars and proper spacing between
+    tasks
+  - Creative solution using SVG injection to implement Gantt chart in pygal (which
+    lacks native Gantt support)
+  - Good use of custom Style for consistent theming and proper font sizing at 4800x2700
+  - Realistic software development project data with logical task sequences and overlaps
+  - Clean color-coded categories with legend for easy interpretation
+  weaknesses:
+  - Legend color boxes are relatively small compared to the large canvas size
+  - Missing current date vertical line marker mentioned in spec notes as a consideration
+  - Red and green colors used together could be improved for colorblind accessibility