Diagram generator

Diagram Generator

Turn any input into a clean diagram. Supports the full Mermaid diagram set so one skill handles flowcharts, sequence diagrams, state machines, ER diagrams, journeys, mindmaps, timelines, gantt charts, sankey, quadrants, class diagrams, and git graphs.

Inputs

• Ticket / PRD / spec — read it and pick the right diagram type.
• File path — read the file and diagram from contents.
• Verbal description — diagram directly from the prompt.
• Existing diagram — refactor / restyle / fix.

Workflow

Step 1 — Pick the diagram type

If the input is unambiguous, pick directly. Otherwise ask which fits best:

User intent	Mermaid type
Process flow, decision tree, "if X then Y"	flowchart
API calls, message passing, who-talks-to-whom over time	sequenceDiagram
Object lifecycle, finite-state machine	stateDiagram-v2
Database schema, entities + relationships	erDiagram
End-to-end customer/user experience with sentiment	journey
Hierarchical brainstorm, taxonomy, idea tree	mindmap
Dated events, milestones	timeline
Project schedule with durations + dependencies	gantt
Flow of quantity between sources/targets	sankey-beta
2x2 categorization (importance/urgency, effort/impact)	quadrantChart
OOP class hierarchy with methods	classDiagram
Git branching strategy	gitGraph

Step 2 — Extract the diagram content

From the input, pull out the right primitives for the chosen type:

• Flowchart: decision points, actions, states, edge cases, scenarios.
• Sequence: actors, messages, ordering, async vs sync.
• State: states, transitions, triggers, terminal states.
• ER: entities, attributes, relationships, cardinality.
• Journey: stages, tasks per stage, sentiment scores.
• Mindmap: root concept + branches.
• Timeline: sections + events per section.
• Gantt: sections + tasks with dates/durations + dependencies.
• Sankey: source → target → value rows.
• Quadrant: axis labels + items with (x, y) scores.
• Class: classes + members + inheritance/composition.
• GitGraph: branches, commits, merges.

If multiple distinct scenarios exist (e.g., happy path + error path), produce SEPARATE diagrams rather than one mega-diagram.

Step 3 — Emit Mermaid syntax

Always wrap output in:

```mermaid
{type-specific code}
```

Follow the type-specific rules below.

Step 4 — Offer a preview file (optional)

After printing the Mermaid block, ask: "Want me to generate a single-file HTML preview that renders this locally?"

If yes, write diagram.html to the current directory using the template under "HTML preview template" below.

Type-specific rules

Flowchart

• Line breaks in labels: ALWAYS <br>, NEVER \n.
• Node shapes:
  • ([text]) — start / end (stadium)
  • [text] — action / process
  • {text} — decision
  • [[text]] — subroutine
  • [(text)] — database
  • ((text)) — circle (event)
• Direction: flowchart TD for processes, flowchart LR for timelines/pipelines.
• Edge labels: -- text --> for conditional paths.
• Colors (apply via style lines at the end):
  • fill:#4CAF50,color:#fff — success / start (green)
  • fill:#EF5350,color:#fff — error / blocked (red)
  • fill:#FF9800,color:#fff — warning / modified (orange)
  • fill:#2196F3,color:#fff — informational (blue)
• Subgraphs: group related items with subgraph Name ... end.
• No special characters (quotes, colons, pipes) inside node labels.
• Node text: max 4 lines.

Sequence diagram

• Use participant X as Display Name to set readable labels.
• Use ->> for sync, -->> for async, --x for failed.
• Use Note over X,Y: text for callouts.
• Use loop, alt / else, par, opt for control flow.
• Use activate X / deactivate X to show lifelines.

State diagram (v2)

• Use [*] --> StateName for start, StateName --> [*] for end.
• Use state ChoiceName <<choice>> for branching.
• Use composite states for nesting: state Group { ... }.

ER diagram

• Cardinality glyphs: ||--o{ (one-to-many), }o--o{ (many-to-many), ||--|| (one-to-one).
• Use lowercase plural entity names.
• Attribute lines: {type} attributeName "comment".

Journey

• Three columns per task: Task name: <score 1-5>: Actor1, Actor2.
• Group tasks under section Name.

Mindmap

• Indentation defines hierarchy.
• Node shapes: ((text)), [text], (text), {{text}}.

Timeline

• section Name then events as : YYYY : event text.

Gantt

• Always start with dateFormat YYYY-MM-DD.
• Tasks: Name :status, id, start, duration or Name :status, id, after otherId, duration.
• Status keywords: done, active, crit.

Sankey-beta

• One source-target-value row per line: Source,Target,42.
• No header. Comma-separated. Values numeric.

Quadrant chart

• Required: title, x-axis Low --> High, y-axis Low --> High, four quadrant labels.
• Items: Item Name: [x, y] where x and y are 0-1.

Class diagram

• Class : +method() for public, - private, # protected.
• Relationships: <|-- inherits, *-- composition, o-- aggregation, --> association.

GitGraph

• commit, branch name, checkout name, merge name.
• Order matters; reads top-to-bottom.

Output structure

## {Diagram Title}

**Summary:** {one-line description of what the diagram shows}

```mermaid
{code}

Key callouts:

• {anything non-obvious about the diagram}

If multiple diagrams (e.g., happy + error paths), use a header per diagram and render each separately.

## HTML preview template

When the user asks for a local preview, write this single file (no build, no npm; mermaid loaded from CDN):

```html
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>Diagram preview</title>
<style>
  body{margin:0;padding:48px;font:16px/1.5 system-ui,sans-serif;background:#F5F1EA;color:#15140F}
  .wrap{max-width:1100px;margin:0 auto}
  h1{font-weight:500;font-size:28px;margin:0 0 24px}
  .mermaid{background:#fff;border:1px solid #D8D2C5;border-radius:10px;padding:24px;overflow:auto}
</style>
</head>
<body>
  <div class="wrap">
    <h1>{TITLE}</h1>
    <pre class="mermaid">
{MERMAID_CODE}
    </pre>
  </div>
  <script type="module">
    import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@10/dist/mermaid.esm.min.mjs';
    mermaid.initialize({ startOnLoad: true, theme: 'base', themeVariables: {
      primaryColor: '#F5F1EA', primaryTextColor: '#15140F', primaryBorderColor: '#15140F',
      lineColor: '#15140F', fontFamily: 'system-ui, sans-serif'
    }});
  </script>
</body>
</html>

Replace {TITLE} and {MERMAID_CODE} with the actual content. Preview opens directly in the browser (file:// works; no server needed).

Anti-Patterns

• Don't pick the wrong diagram type to "force fit" the input — ask if it's ambiguous.
• Don't cram multiple scenarios into one diagram; split them.
• Don't use \n for line breaks in flowchart node labels — always <br>.
• Don't put quotes, colons, or pipes inside node labels (they break Mermaid).
• Don't skip the style lines on flowcharts — color coding makes diagrams scannable.
• Don't reach for D3, ReactFlow, or custom HTML; Mermaid is the right tool for almost every case. (If the user wants the magazine-grade animated process flow, point them to the interactive-flowchart-builder skill instead.)
• Don't auto-generate the HTML preview unless the user asks — most users just want the Mermaid block.

Quality Checklist

• Picked the right diagram type for the input (flowchart vs sequence vs state vs ...).
• Mermaid syntax parses cleanly (no unescaped special chars in labels).
• Distinct scenarios split into separate diagrams.
• Color/style applied where the diagram type supports it (flowchart, sequence highlights, journey scores).
• Node text is concise (≤4 lines).
• Summary line + key callouts included with the diagram.
• If preview requested, single-file HTML written and opens directly in browser.