spec: add spectrum-basic specification (#2932)

## New Specification: `spectrum-basic`

Related to #2926

---

### specification.md

# spectrum-basic: Frequency Spectrum Plot

## Description

A frequency spectrum plot displays signal amplitude or power across a
range of frequencies, showing the frequency domain representation of
time-series data. This visualization reveals the frequency components
present in a signal, making it essential for identifying dominant
frequencies, harmonics, and noise characteristics. It is fundamental in
signal processing, audio engineering, and vibration analysis.

## Applications

- Analyzing audio signals to identify frequency content such as musical
notes, speech formants, or noise interference
- Detecting machinery vibration patterns and identifying potential
mechanical faults based on characteristic frequencies
- Examining electrical signals to find interference frequencies or
verify filter performance

## Data

- `frequency` (numeric) - frequency values in Hz, typically from FFT
computation
- `amplitude` (numeric) - signal amplitude or magnitude at each
frequency, often in dB or linear scale
- Size: 256-4096 frequency bins (typical FFT sizes)
- Example: FFT output of a synthetic signal with multiple frequency
components

## Notes

- Use logarithmic scale for frequency axis when spanning wide frequency
ranges
- Power spectral density (dB scale) is common for comparing signals with
different amplitudes
- Include clear axis labels with units (Hz for frequency, dB or linear
for amplitude)
- Consider highlighting peak frequencies or annotating dominant
components

---

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---
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plots/spectrum-basic/specification.md

@@ -0,0 +1,25 @@
+# spectrum-basic: Frequency Spectrum Plot
+
+## Description
+
+A frequency spectrum plot displays signal amplitude or power across a range of frequencies, showing the frequency domain representation of time-series data. This visualization reveals the frequency components present in a signal, making it essential for identifying dominant frequencies, harmonics, and noise characteristics. It is fundamental in signal processing, audio engineering, and vibration analysis.
+
+## Applications
+
+- Analyzing audio signals to identify frequency content such as musical notes, speech formants, or noise interference
+- Detecting machinery vibration patterns and identifying potential mechanical faults based on characteristic frequencies
+- Examining electrical signals to find interference frequencies or verify filter performance
+
+## Data
+
+- `frequency` (numeric) - frequency values in Hz, typically from FFT computation
+- `amplitude` (numeric) - signal amplitude or magnitude at each frequency, often in dB or linear scale
+- Size: 256-4096 frequency bins (typical FFT sizes)
+- Example: FFT output of a synthetic signal with multiple frequency components
+
+## Notes
+
+- Use logarithmic scale for frequency axis when spanning wide frequency ranges
+- Power spectral density (dB scale) is common for comparing signals with different amplitudes
+- Include clear axis labels with units (Hz for frequency, dB or linear for amplitude)
+- Consider highlighting peak frequencies or annotating dominant components

plots/spectrum-basic/specification.yaml

@@ -0,0 +1,29 @@
+# Specification-level metadata for spectrum-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: spectrum-basic
+title: Frequency Spectrum Plot
+
+# Specification tracking
+created: 2025-12-31T05:17:32Z
+updated: null
+issue: 2926
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/concepts/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - spectrum
+    - line
+  data_type:
+    - numeric
+    - continuous
+    - frequency
+  domain:
+    - general
+    - engineering
+    - science
+  features:
+    - basic
+    - signal-analysis