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Mermaid Diagrams

Create effective, syntactically valid mermaid diagrams for product documents.

When to Use

• Creating mermaid diagrams for PRDs, specs, roadmaps, or stakeholder presentations
• Choosing which of 15 diagram types fits a specific communication need
• Debugging mermaid code that won't render or renders incorrectly
• Reviewing diagrams for clarity, accuracy, and accessibility

When NOT to Use

• Exporting diagrams to image files (PNG/SVG) . that's a rendering tool concern
• Using non-mermaid diagramming tools (Figma, Lucidchart, draw.io)
• Creating purely decorative visuals with no informational purpose

The Cardinal Rule

Don't diagram what a list can say.

Diagrams earn their place when they reveal relationships, branching, or flow that prose flattens. Before creating any diagram, ask:

Does this show branching, relationships, or flow that a list or table would flatten?

• Yes → proceed with a diagram
• No → use a numbered list, bullet list, or table instead

A 5-step linear process is a list. A 5-step process with two decision points and a retry loop is a diagram.

Diagram Selection Guide

I need to show...	Use	Also consider
A decision or approval process	Flowchart	State
Multi-service or multi-party interactions	Sequence	Flowchart
Feature lifecycle or status transitions	State	Flowchart
Work stages or pipeline status	Kanban	State
Release or sprint timeline with dependencies	Gantt	Timeline
Version history or chronological milestones	Timeline	Gantt
2D prioritization (effort/impact, risk/value)	Quadrant	.
Allocation breakdown or composition	Pie	Treemap
Problem decomposition or brainstorming	Mindmap	.
Domain models or data relationships	ER	Class
API or object contracts	Class	ER
System topology or infrastructure	Architecture	Flowchart
Flow quantities or budget allocation	Sankey	Pie
Hierarchical proportional data	Treemap	Pie
Trends or time-series metrics	XY-Chart	.

For worked examples organized by PM task, see references/pm-use-cases.md. For full syntax and options per type, see references/diagram-catalog.md.

Syntax Validity Principles

Six rules that prevent most rendering failures:

1. Quote labels . Any label containing spaces, parentheses, brackets, colons, commas, or reserved words must be quoted with double quotes
2. Escape special characters . Characters with mermaid or markdown meaning (>, <, - at line start, #) need escaping or quoting
3. Declare before referencing . Define a node before using it in an edge; referencing an undeclared node causes silent failures in some types
4. Respect limits . Each diagram type has a maximum node/participant count beyond which readability collapses (see references/diagram-catalog.md for per-type limits)
5. Comment your intent . Use %% comments to document non-obvious choices (why this layout direction, why this grouping)
6. Test before shipping . Paste into a mermaid renderer (mermaid.live, VS Code preview, or your target environment) and verify it renders correctly

For the complete syntax reference, see references/syntax-guide.md.

Instructions

1. Identify what you're communicating . What relationship, flow, hierarchy, or proportion needs to be visible? Who is the audience?
2. Apply the cardinal rule . Confirm a diagram adds value over a list or table
3. Select a diagram type . Use the selection guide above, browse references/pm-use-cases.md by task, or browse references/diagram-catalog.md by type
4. Plan the diagram . Fill out the planning worksheet in references/TEMPLATE.md: purpose, audience, node inventory, type rationale
5. Write the mermaid code . Follow references/syntax-guide.md rules; start with the minimal syntax example from references/diagram-catalog.md and expand
6. Validate . Run through the quality checklist below
7. Embed . Place the validated mermaid code block in your document

Output Contract

• Planning artifact: A completed diagram planning worksheet (references/TEMPLATE.md)
• Final output: A syntactically valid mermaid code block embedded in the target document
• Quality gate: All items in the quality checklist pass

Quality Checklist

• Diagram renders without error in target environment
• Cardinal rule satisfied . a list or table would not communicate this more clearly
• No linear sequences without branching, relationships, or hierarchy
• All labels with spaces or special characters are properly quoted
• Special characters escaped where needed
• Node/participant count within type-specific limits
• Colors are accessible (WCAG AA 3:1 contrast minimum, black text on light backgrounds)
• Color is never the sole differentiator . shapes and labels also distinguish elements
• Diagram has a descriptive title or surrounding prose context
• %% comments document any non-obvious layout or grouping choices

References

File	Purpose
references/TEMPLATE.md	Diagram planning worksheet . fill out before writing mermaid code
references/EXAMPLE.md	Worked example: PM creating 4 diagrams for a product launch
references/diagram-catalog.md	All 15 diagram types: syntax, PM examples, limits, pitfalls
references/pm-use-cases.md	PM task → diagram type mapping with mini worked examples
references/syntax-guide.md	Complete syntax validity rules, escaping, styling, and validation checklist