spec: add line-timeseries-rolling specification

Created from issue #2786

Author: github-actions[bot]

plots/line-timeseries-rolling/specification.md

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+# line-timeseries-rolling: Time Series with Rolling Average Overlay
+
+## Description
+
+A time series plot that displays raw data points alongside a smoothed rolling average (moving average) line. The raw data shows actual observations while the rolling average reveals underlying trends by reducing noise and short-term fluctuations. This dual-layer visualization is essential for trend identification, making patterns visible that might be obscured by day-to-day volatility.
+
+## Applications
+
+- Analyzing stock price movements with a moving average to identify buy/sell signals and trend reversals
+- Monitoring website traffic or user engagement metrics with smoothed trends for seasonal pattern detection
+- Tracking sensor data (temperature, humidity) with noise reduction to reveal true environmental trends
+
+## Data
+
+- `date` (datetime) - Timestamp values representing points in time
+- `value` (numeric) - Raw measurements or observations at each timestamp
+- `rolling_avg` (numeric) - Computed rolling average (e.g., 7-day, 30-day window)
+- Size: 50-500 points (enough data for meaningful rolling window calculation)
+- Example: Daily stock closing prices with 20-day moving average, hourly temperature readings with 24-hour rolling mean
+
+## Notes
+
+- Use a lighter, semi-transparent style for raw data (thin line or markers with alpha)
+- Display the rolling average as a prominent, smooth line in a contrasting color
+- Include a legend clearly distinguishing "Raw Data" from "Rolling Average (N-day)"
+- Consider showing the window size in the legend or title (e.g., "7-Day Rolling Average")
+- Grid lines on both axes improve readability of underlying values
+- The rolling average line will be shorter than raw data due to window requirements

plots/line-timeseries-rolling/specification.yaml

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+# Specification-level metadata for line-timeseries-rolling
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: line-timeseries-rolling
+title: Time Series with Rolling Average Overlay
+
+# Specification tracking
+created: 2025-12-30T17:31:12Z
+updated: null
+issue: 2786
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/concepts/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - line
+    - timeseries
+  data_type:
+    - timeseries
+    - numeric
+    - continuous
+  domain:
+    - general
+    - finance
+    - statistics
+  features:
+    - smoothing
+    - rolling-average
+    - trend-analysis
+    - overlay