spec: add indicator-sma specification (#3657)

## New Specification: `indicator-sma`

Related to #3651

---

### specification.md

# indicator-sma: Simple Moving Average (SMA) Indicator Chart

## Description

A Simple Moving Average (SMA) chart displays price or value data with
one or more SMA overlays, typically showing short, medium, and long-term
periods (e.g., 20, 50, 200-day). Each SMA line smooths out price
fluctuations by averaging the last N data points, revealing underlying
trends at different time scales. This multi-period overlay is
fundamental to technical analysis, helping traders identify trend
direction, support/resistance levels, and potential crossover signals.

## Applications

- Identifying trend direction by comparing price position relative to
50-day and 200-day SMAs in stock analysis
- Detecting golden cross (50-day crosses above 200-day) and death cross
signals in cryptocurrency trading
- Analyzing short-term momentum versus long-term trend using
20/50/200-day SMA combinations
- Smoothing noisy time series data to reveal underlying patterns in
sensor or metrics monitoring

## Data

- `date` (datetime) - Trading date or timestamp for each period
- `close` (numeric) - Closing price or value for each period
- `sma_short` (numeric) - Short-term SMA (e.g., 20-period)
- `sma_medium` (numeric) - Medium-term SMA (e.g., 50-period)
- `sma_long` (numeric) - Long-term SMA (e.g., 200-period)
- Size: 250-500 periods to show meaningful long-term SMA behavior
- Example: Daily stock closing prices with 20, 50, and 200-day SMAs over
one year of trading

## Notes

- Display the price/close line prominently, typically as a solid line or
with subtle markers
- Each SMA should use a distinct color with consistent styling across
all implementations
- Include a legend showing the period for each SMA (e.g., "SMA 20", "SMA
50", "SMA 200")
- Longer SMAs will have more initial null values due to the calculation
window
- Standard periods are 20/50/200 for daily data, but other combinations
like 10/20/50 are also common
- Grid lines on the y-axis improve price level readability

---

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---
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plots/indicator-sma/specification.md

@@ -0,0 +1,31 @@
+# indicator-sma: Simple Moving Average (SMA) Indicator Chart
+
+## Description
+
+A Simple Moving Average (SMA) chart displays price or value data with one or more SMA overlays, typically showing short, medium, and long-term periods (e.g., 20, 50, 200-day). Each SMA line smooths out price fluctuations by averaging the last N data points, revealing underlying trends at different time scales. This multi-period overlay is fundamental to technical analysis, helping traders identify trend direction, support/resistance levels, and potential crossover signals.
+
+## Applications
+
+- Identifying trend direction by comparing price position relative to 50-day and 200-day SMAs in stock analysis
+- Detecting golden cross (50-day crosses above 200-day) and death cross signals in cryptocurrency trading
+- Analyzing short-term momentum versus long-term trend using 20/50/200-day SMA combinations
+- Smoothing noisy time series data to reveal underlying patterns in sensor or metrics monitoring
+
+## Data
+
+- `date` (datetime) - Trading date or timestamp for each period
+- `close` (numeric) - Closing price or value for each period
+- `sma_short` (numeric) - Short-term SMA (e.g., 20-period)
+- `sma_medium` (numeric) - Medium-term SMA (e.g., 50-period)
+- `sma_long` (numeric) - Long-term SMA (e.g., 200-period)
+- Size: 250-500 periods to show meaningful long-term SMA behavior
+- Example: Daily stock closing prices with 20, 50, and 200-day SMAs over one year of trading
+
+## Notes
+
+- Display the price/close line prominently, typically as a solid line or with subtle markers
+- Each SMA should use a distinct color with consistent styling across all implementations
+- Include a legend showing the period for each SMA (e.g., "SMA 20", "SMA 50", "SMA 200")
+- Longer SMAs will have more initial null values due to the calculation window
+- Standard periods are 20/50/200 for daily data, but other combinations like 10/20/50 are also common
+- Grid lines on the y-axis improve price level readability

plots/indicator-sma/specification.yaml

@@ -0,0 +1,28 @@
+# Specification-level metadata for indicator-sma
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: indicator-sma
+title: Simple Moving Average (SMA) Indicator Chart
+
+# Specification tracking
+created: 2026-01-11T00:09:18Z
+updated: null
+issue: 3651
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - line
+    - indicator
+  data_type:
+    - timeseries
+    - numeric
+  domain:
+    - finance
+    - general
+  features:
+    - multi
+    - overlay
+    - trend