spec: add heatmap-loss-triangle specification (#4602)

## New Specification: `heatmap-loss-triangle`

Related to #4581

---

### specification.md

# heatmap-loss-triangle: Actuarial Loss Development Triangle

## Description

A triangular matrix visualization showing cumulative insurance claim
payments developing over time. Rows represent accident/origin years,
columns represent development periods (e.g., 1-10 years), with the
upper-left triangle displaying actual observed cumulative claims and the
lower-right triangle showing projected/estimated values (IBNR). This
plot is essential for actuarial reserving, enabling analysts to
visualize the chain-ladder method and identify development patterns in
loss data.

## Applications

- Insurance reserving: visualizing cumulative claims development to
estimate IBNR (Incurred But Not Reported) reserves using the
chain-ladder method
- Reinsurance pricing: assessing reserve adequacy and treaty pricing by
examining historical loss development patterns
- Financial reporting: preparing loss reserve disclosures and
communicating reserve estimates to stakeholders

## Data

- `accident_year` (integer) - The origin/accident year when claims
occurred (e.g., 2015-2024)
- `development_period` (integer) - The development period in years from
origin (e.g., 1-10)
- `cumulative_amount` (float) - Cumulative claim payments at each
development stage
- `is_projected` (boolean) - Whether the value is actual (observed) or
projected (estimated)
- `development_factor` (float) - Age-to-age development factors between
consecutive periods
- Size: 10 accident years x 10 development periods (triangular: ~55
actual + ~45 projected cells)
- Example: Cumulative paid claims triangle with accident years 2015-2024
and development periods 1-10

## Notes

- The upper-left triangle contains actual/observed values; the
lower-right triangle contains projected values
- Use distinct visual styling (e.g., different background colors or
hatching) to clearly differentiate actual from projected cells
- The diagonal from top-right to bottom-left represents the latest
evaluation date
- Annotate each cell with the cumulative amount value (formatted with
thousands separator)
- Display age-to-age development factors between column headers or as a
separate row below the triangle
- Color intensity should encode the magnitude of cumulative amounts
- Include a clear legend distinguishing actual vs projected regions
- Row labels show accident/origin years; column headers show development
periods

---

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---
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plots/heatmap-loss-triangle/specification.md

@@ -0,0 +1,32 @@
+# heatmap-loss-triangle: Actuarial Loss Development Triangle
+
+## Description
+
+A triangular matrix visualization showing cumulative insurance claim payments developing over time. Rows represent accident/origin years, columns represent development periods (e.g., 1-10 years), with the upper-left triangle displaying actual observed cumulative claims and the lower-right triangle showing projected/estimated values (IBNR). This plot is essential for actuarial reserving, enabling analysts to visualize the chain-ladder method and identify development patterns in loss data.
+
+## Applications
+
+- Insurance reserving: visualizing cumulative claims development to estimate IBNR (Incurred But Not Reported) reserves using the chain-ladder method
+- Reinsurance pricing: assessing reserve adequacy and treaty pricing by examining historical loss development patterns
+- Financial reporting: preparing loss reserve disclosures and communicating reserve estimates to stakeholders
+
+## Data
+
+- `accident_year` (integer) - The origin/accident year when claims occurred (e.g., 2015-2024)
+- `development_period` (integer) - The development period in years from origin (e.g., 1-10)
+- `cumulative_amount` (float) - Cumulative claim payments at each development stage
+- `is_projected` (boolean) - Whether the value is actual (observed) or projected (estimated)
+- `development_factor` (float) - Age-to-age development factors between consecutive periods
+- Size: 10 accident years x 10 development periods (triangular: ~55 actual + ~45 projected cells)
+- Example: Cumulative paid claims triangle with accident years 2015-2024 and development periods 1-10
+
+## Notes
+
+- The upper-left triangle contains actual/observed values; the lower-right triangle contains projected values
+- Use distinct visual styling (e.g., different background colors or hatching) to clearly differentiate actual from projected cells
+- The diagonal from top-right to bottom-left represents the latest evaluation date
+- Annotate each cell with the cumulative amount value (formatted with thousands separator)
+- Display age-to-age development factors between column headers or as a separate row below the triangle
+- Color intensity should encode the magnitude of cumulative amounts
+- Include a clear legend distinguishing actual vs projected regions
+- Row labels show accident/origin years; column headers show development periods

plots/heatmap-loss-triangle/specification.yaml

@@ -0,0 +1,22 @@
+spec_id: heatmap-loss-triangle
+title: Actuarial Loss Development Triangle
+
+# Specification tracking
+created: "2026-03-06T19:53:43Z"
+updated: null
+issue: 4581
+suggested: MarkusNeusinger
+
+# Classification tags
+tags:
+  plot_type:
+    - heatmap
+  data_type:
+    - numeric
+    - matrix
+  domain:
+    - finance
+  features:
+    - annotated
+    - color-mapped
+    - cumulative