feat(plotnine): implement area-basic

Add basic area chart implementation for plotnine library.

- Creates spec file: specs/area-basic.md
- Creates implementation: plots/plotnine/area/area-basic/default.py
- Features: filled area with line overlay, customizable colors and transparency
- Uses geom_area() and geom_line() from plotnine
- Includes input validation, type hints, and Google-style docstrings

Author: github-actions[bot]

plots/plotnine/area/area-basic/default.py

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+"""
+area-basic: Basic Area Chart
+Implementation for: plotnine
+Variant: default
+Python: 3.10+
+"""
+
+from typing import TYPE_CHECKING, Optional
+
+import numpy as np
+import pandas as pd
+from plotnine import aes, element_line, element_text, geom_area, geom_line, ggplot, labs, theme, theme_minimal
+
+
+if TYPE_CHECKING:
+    from plotnine import ggplot as GGPlot
+
+
+def create_plot(
+    data: pd.DataFrame,
+    x: str,
+    y: str,
+    title: Optional[str] = None,
+    xlabel: Optional[str] = None,
+    ylabel: Optional[str] = None,
+    color: str = "steelblue",
+    alpha: float = 0.6,
+    line_color: Optional[str] = None,
+    line_width: float = 1.5,
+    width: int = 16,
+    height: int = 9,
+    **kwargs,
+) -> "GGPlot":
+    """
+    Create a basic area chart showing values over a continuous axis using plotnine (ggplot2 syntax).
+
+    Args:
+        data: Input DataFrame with required columns
+        x: Column name for x-axis values (numeric or datetime)
+        y: Column name for y-axis values (numeric)
+        title: Plot title (optional)
+        xlabel: Custom x-axis label (optional, defaults to x column name)
+        ylabel: Custom y-axis label (optional, defaults to y column name)
+        color: Fill color for the area (default: 'steelblue')
+        alpha: Fill transparency level (default: 0.6)
+        line_color: Color of the top line (default: same as color)
+        line_width: Width of the top line (default: 1.5)
+        width: Figure width in inches (default: 16)
+        height: Figure height in inches (default: 9)
+        **kwargs: Additional parameters for geom_area
+
+    Returns:
+        plotnine ggplot object
+
+    Raises:
+        ValueError: If data is empty
+        KeyError: If required columns not found
+
+    Example:
+        >>> data = pd.DataFrame({
+        ...     'time': [1, 2, 3, 4, 5],
+        ...     'value': [10, 25, 15, 30, 20]
+        ... })
+        >>> plot = create_plot(data, x='time', y='value')
+    """
+    # Input validation
+    if data.empty:
+        raise ValueError("Data cannot be empty")
+
+    # Check required columns
+    for col in [x, y]:
+        if col not in data.columns:
+            available = ", ".join(data.columns)
+            raise KeyError(f"Column '{col}' not found. Available columns: {available}")
+
+    # Use the same color for line if not specified
+    if line_color is None:
+        line_color = color
+
+    # Sort data by x to ensure proper area rendering
+    data_sorted = data.sort_values(by=x).copy()
+
+    # Create the ggplot object with area and line
+    plot = (
+        ggplot(data_sorted, aes(x=x, y=y))
+        + geom_area(fill=color, alpha=alpha, **kwargs)
+        + geom_line(color=line_color, size=line_width)
+        + labs(title=title or "Area Chart", x=xlabel or x, y=ylabel or y)
+        + theme_minimal()
+        + theme(
+            figure_size=(width, height),
+            plot_title=element_text(size=14, weight="bold", ha="center"),
+            axis_title=element_text(size=11),
+            axis_text=element_text(size=10),
+            panel_grid_major=element_line(alpha=0.3, linetype="dashed"),
+            panel_grid_minor=element_line(alpha=0),
+        )
+    )
+
+    return plot
+
+
+if __name__ == "__main__":
+    # Sample data for testing - simulating time series data
+    np.random.seed(42)  # For reproducibility
+
+    # Generate sample time series data (e.g., monthly website visitors)
+    months = pd.date_range(start="2024-01-01", periods=12, freq="MS")
+
+    # Create realistic-looking growth pattern with some variation
+    base_values = np.linspace(1000, 2500, 12)
+    noise = np.random.normal(0, 150, 12)
+    values = base_values + noise
+
+    # Ensure no negative values
+    values = np.maximum(values, 100)
+
+    data = pd.DataFrame(
+        {
+            "Month": range(1, 13),  # Use numeric for simpler plotting
+            "Visitors": values,
+        }
+    )
+
+    # Create plot
+    plot = create_plot(
+        data,
+        x="Month",
+        y="Visitors",
+        title="Monthly Website Visitors (2024)",
+        xlabel="Month",
+        ylabel="Number of Visitors",
+        color="#3498db",
+        alpha=0.5,
+        line_color="#2980b9",
+        line_width=2,
+    )
+
+    # Save for inspection
+    plot.save("plot.png", dpi=300, verbose=False)
+    print("Plot saved to plot.png")

specs/area-basic.md

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+# area-basic: Basic Area Chart
+
+<!--
+Spec Template Version: 1.0.0
+Created: 2025-12-01
+Last Updated: 2025-12-01
+-->
+
+**Spec Version:** 1.0.0
+
+## Description
+
+Create a simple area chart that displays a single filled area beneath a line, ideal for showing how a quantity changes over time or another continuous variable. The filled area emphasizes the magnitude of values and cumulative totals.
+
+## Data Requirements
+
+- **x**: Numeric or datetime column for the x-axis (continuous variable, typically time or sequence)
+- **y**: Numeric column for the y-axis (the values to plot)
+
+## Optional Parameters
+
+- `color`: Fill color for the area (type: string, default: "steelblue")
+- `alpha`: Fill transparency level (type: float 0.0-1.0, default: 0.6)
+- `line_color`: Color of the top line (type: string, default: same as color)
+- `line_width`: Width of the top line (type: float, default: 1.5)
+- `title`: Plot title (type: string, default: None)
+- `xlabel`: Custom x-axis label (type: string, default: column name)
+- `ylabel`: Custom y-axis label (type: string, default: column name)
+- `figsize`: Figure size (type: tuple, default: (16, 9))
+
+## Quality Criteria
+
+- [ ] X and Y axes are labeled with column names (or custom labels if provided)
+- [ ] Grid is visible but subtle with alpha=0.3
+- [ ] Area fill is clearly visible with appropriate transparency
+- [ ] Line on top of area provides clear boundary
+- [ ] No overlapping axis labels or tick marks
+- [ ] Appropriate figure size (16:9 aspect ratio) for readability
+- [ ] Title is centered and clearly readable if provided
+
+## Expected Output
+
+A clean area chart with a filled region showing values over a continuous axis. The area should be filled with a semi-transparent color, bounded by a line at the top edge. The baseline should be at y=0. Grid lines should help with reading values without overpowering the data. All text elements (labels, title, and tick labels) should be legible at standard display sizes.
+
+## Tags
+
+trend, time-series, basic, area, continuous, statistical, exploratory
+
+## Use Cases
+
+- Stock price or market value over time (e.g., portfolio value growth)
+- Website traffic visualization (e.g., daily visitors over months)
+- Resource usage monitoring (e.g., CPU or memory usage over time)
+- Sales trends over time periods (e.g., monthly revenue)
+- Temperature or environmental data over time
+- Population growth or demographic changes