spec: add area-cumulative-flow specification

Author: github-actions[bot]

plots/area-cumulative-flow/specification.md

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+# area-cumulative-flow: Cumulative Flow Diagram for Workflow Analytics
+
+## Description
+
+A cumulative flow diagram (CFD) displays the cumulative count of items in each workflow stage as stacked areas over time. Each band represents a stage (e.g., Backlog, In Progress, Done), and the vertical distance between two adjacent band boundaries shows the number of items currently in that stage (work-in-progress). This is a key visualization in Lean and Agile project management for identifying bottlenecks, monitoring throughput, and assessing flow efficiency across a delivery pipeline.
+
+## Applications
+
+- Monitoring Kanban board throughput and cycle time to assess team delivery performance
+- Identifying bottlenecks in software development pipelines by spotting widening or narrowing bands
+- Tracking manufacturing workflow stages over time to optimize production flow
+- Visualizing support ticket lifecycle stages to measure resolution efficiency
+
+## Data
+
+- `date` (date) - time axis representing the reporting period
+- `stage` (categorical, ordered) - workflow stage name in process order (e.g., Backlog, Analysis, Development, Testing, Done)
+- `count` (int) - cumulative number of items that have entered or passed through that stage by the given date
+- Size: 30-365 days, 4-8 stages
+- Example: daily cumulative item counts across a Kanban board with stages Backlog, In Progress, Review, Done over 90 days
+
+## Notes
+
+- Stages must be stacked in workflow order: earliest stage (e.g., Backlog) on top, latest stage (e.g., Done) on bottom
+- The vertical distance between two adjacent band boundaries represents the number of items currently in that stage (WIP)
+- Cumulative counts are monotonically non-decreasing; each stage's count should always be greater than or equal to the count of the next stage in the workflow
+- Widening bands indicate growing WIP or bottlenecks; narrowing bands indicate stages draining faster than they fill
+- Use a sequential or distinct color palette where earlier stages are visually distinguishable from later stages
+- Include a legend identifying each workflow stage
+- X-axis should display date labels at reasonable intervals

plots/area-cumulative-flow/specification.yaml

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+# Specification-level metadata for area-cumulative-flow
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: area-cumulative-flow
+title: Cumulative Flow Diagram for Workflow Analytics
+
+# Specification tracking
+created: "2026-04-11T20:22:20Z"
+updated: null
+issue: 5239
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - area
+    - stacked
+  data_type:
+    - timeseries
+    - categorical
+    - numeric
+  domain:
+    - business
+    - technology
+  features:
+    - stacked
+    - cumulative
+    - flow
+    - temporal