draw-io-diagram-generator

Draw.io Diagram Generator

This skill enables you to generate, edit, and validate draw.io (.drawio) diagram files with correct mxGraph XML structure. All generated files open immediately in the Draw.io VS Code extension (hediet.vscode-drawio) without any manual fixes required. You can also open the files in the draw.io web app or desktop app if you prefer.

1. When to Use This Skill

Trigger phrases (load this skill when you see these)

• "create a diagram", "draw a flowchart", "generate an architecture diagram"
• "design a sequence diagram", "make a UML class diagram", "build an ER diagram"
• "add a .drawio file", "update the diagram", "visualise the flow"
• "document the architecture", "show the data model", "diagram the service interactions"
• Any request to produce or modify a .drawio, .drawio.svg, or .drawio.png file

Supported diagram types

2. Prerequisites

• If running with VS Code integration enabled, Install the drawio extension: draw.io VS Code extension — hediet.vscode-drawio (extension id). Install with:

  ext install hediet.vscode-drawio
• Supported file extensions: .drawio, .drawio.svg, .drawio.png
• Python 3.8+ (optional) — for the validation and shape-insertion scripts in scripts/

3. Step-by-Step Agent Workflow

Follow these steps in order for every diagram generation task.

Step 1 — Understand the Request

Ask or infer:

1. Diagram type — What kind of diagram? (flowchart, architecture, UML, ER, sequence, network...)
2. Entities / actors — What are the main components, actors, classes, or tables?
3. Relationships — How are they connected? What direction? What cardinality?
4. Output path — Where should the .drawio file be saved?
5. Existing file — Are we creating new or editing an existing file?

If the request is ambiguous, infer the most sensible diagram type from context (e.g. "show the tables" → ER diagram, "show how the API call flows" → sequence diagram).

Step 2 — Select a Template or Start Fresh

• Use a template when the diagram type matches one in assets/templates/. Copy the template structure and replace placeholder values.
• Start fresh for novel layouts. Begin with the minimal valid skeleton:

<!-- Set modified="" to the current ISO 8601 timestamp when generating a new file -->
<mxfile host="Electron" modified="" version="26.0.0">
  <diagram id="page-1" name="Page-1">
    <mxGraphModel dx="1422" dy="762" grid="1" gridSize="10" guides="1"
                  tooltips="1" connect="1" arrows="1" fold="1"
                  page="1" pageScale="1" pageWidth="1169" pageHeight="827"
                  math="0" shadow="0">
      <root>
        <mxCell id="0" />
        <mxCell id="1" parent="0" />
        <!-- Your cells go here -->
      </root>
    </mxGraphModel>
  </diagram>
</mxfile>

Rule: ids 0 and 1 are ALWAYS required and must be the first two cells. Never reuse them.

Step 3 — Plan the Layout

Before generating XML, sketch the logical placement:

• Organise into rows or tiers (use swimlanes for layers)
• Horizontal spacing: 40–60px between same-row shapes
• Vertical spacing: 80–120px between tier rows
• Standard shape size: 120x60 px for process boxes, 160x80 px for swimlanes
• Default canvas: A4 landscape = 1169 x 827 px

Step 4 — Generate the mxGraph XML

Vertex cell (every shape):

<mxCell id="unique-id" value="Label"
        style="rounded=1;whiteSpace=wrap;html=1;fillColor=#dae8fc;strokeColor=#6c8ebf;"
        vertex="1" parent="1">
  <mxGeometry x="100" y="100" width="120" height="60" as="geometry" />
</mxCell>

Edge cell (every connector):

<mxCell id="edge-id" value="Label (optional)"
        style="edgeStyle=orthogonalEdgeStyle;html=1;"
        edge="1" source="source-id" target="target-id" parent="1">
  <mxGeometry relative="1" as="geometry" />
</mxCell>

Critical rules:

• Every cell id must be globally unique within the file
• Every vertex must have an mxGeometry child with x, y, width, height, as="geometry"
• Every edge must have source and target matching existing vertex ids — exception: floating edges (e.g. sequence diagram lifelines) use sourcePoint/targetPoint inside <mxGeometry> instead; see §4 Sequence Diagram
• Every cell's parent must reference an existing cell id
• Use html=1 in style when the label contains HTML (<b>, <i>, <br>)
• Escape XML special characters in labels: & => &, < => <, > => >

Step 5 — Apply Correct Styles

Use the standard semantic color palette for consistency:

Common style strings by diagram type:

# Rounded process box (flowchart)
rounded=1;whiteSpace=wrap;html=1;fillColor=#dae8fc;strokeColor=#6c8ebf;
 
# Decision diamond
rhombus;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;
 
# Start/End terminal
ellipse;whiteSpace=wrap;html=1;fillColor=#d5e8d4;strokeColor=#82b366;
 
# Database cylinder
shape=mxgraph.flowchart.database;whiteSpace=wrap;html=1;fillColor=#f8cecc;strokeColor=#b85450;
 
# Swimlane container (tier)
swimlane;startSize=30;fillColor=#dae8fc;strokeColor=#6c8ebf;fontStyle=1;
 
# UML class box
swimlane;fontStyle=1;align=center;startSize=40;fillColor=#dae8fc;strokeColor=#6c8ebf;
 
# Interface / stereotype box
swimlane;fontStyle=3;align=center;startSize=40;fillColor=#f5f5f5;strokeColor=#666666;
 
# ER table container
shape=table;startSize=30;container=1;collapsible=1;childLayout=tableLayout;
 
# Orthogonal connector
edgeStyle=orthogonalEdgeStyle;html=1;
 
# ER relationship (crow's foot)
edgeStyle=entityRelationEdgeStyle;html=1;endArrow=ERmany;startArrow=ERone;

See references/style-reference.md for the complete style key catalog and references/shape-libraries.md for all shape library names.

Step 6 — Save and Validate

1. Write the file to the requested path with .drawio extension
2. Run the validator (optional but recommended):

   python .github/skills/draw-io-diagram-generator/scripts/validate-drawio.py <path-to-file.drawio>
3. Tell the user how to open the file:

   "Open <filename> in VS Code — it will render automatically with the draw.io extension. You can use draw.io's web app or desktop app as well if you prefer."

4. Provide a brief description of what is in the diagram so the user knows what to expect.

4. Diagram-Type Recipes

Flowchart

Key elements: Start (ellipse) => Process (rounded rectangle) => Decision (diamond) => End (ellipse)

<!-- Start node -->
<mxCell id="start" value="Start"
        style="ellipse;whiteSpace=wrap;html=1;fillColor=#d5e8d4;strokeColor=#82b366;"
        vertex="1" parent="1">
  <mxGeometry x="500" y="80" width="120" height="60" as="geometry" />
</mxCell>
 
<!-- Process -->
<mxCell id="p1" value="Process Step"
        style="rounded=1;whiteSpace=wrap;html=1;fillColor=#dae8fc;strokeColor=#6c8ebf;"
        vertex="1" parent="1">
  <mxGeometry x="500" y="200" width="120" height="60" as="geometry" />
</mxCell>
 
<!-- Decision -->
<mxCell id="d1" value="Condition?"
        style="rhombus;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;"
        vertex="1" parent="1">
  <mxGeometry x="460" y="320" width="200" height="100" as="geometry" />
</mxCell>
 
<!-- Arrow: start to p1 -->
<mxCell id="e1" value=""
        style="edgeStyle=orthogonalEdgeStyle;html=1;"
        edge="1" source="start" target="p1" parent="1">
  <mxGeometry relative="1" as="geometry" />
</mxCell>

Architecture Diagram (3-tier)

Use swimlane containers for each tier. All service boxes are children of their swimlane.

<!-- Tier swimlane -->
<mxCell id="tier1" value="Client Layer"
        style="swimlane;startSize=30;fillColor=#dae8fc;strokeColor=#6c8ebf;fontStyle=1;"
        vertex="1" parent="1">
  <mxGeometry x="60" y="100" width="1050" height="130" as="geometry" />
</mxCell>
 
<!-- Service inside tier (parent="tier1", coords are relative to tier) -->
<mxCell id="webapp" value="Web App"
        style="rounded=1;whiteSpace=wrap;html=1;fillColor=#dae8fc;strokeColor=#6c8ebf;"
        vertex="1" parent="tier1">
  <mxGeometry x="80" y="40" width="120" height="60" as="geometry" />
</mxCell>

Connectors between tiers use absolute coordinates with parent="1".

Sequence Diagram

Key elements: Actors (top), Lifelines (dashed vertical lines), Activation boxes, Message arrows.

• Lifelines: edge="1" with endArrow=none and dashed=1, no source/target — use sourcePoint/targetPoint in geometry
• Synchronous message: endArrow=block;endFill=1
• Return message: endArrow=open;endFill=0;dashed=1
• Self-call: loop the edge via two Array points to the right and back

Minimal XML snippet:

<!-- Actor (stick figure) -->
<mxCell id="actorA" value="Client"
        style="shape=mxgraph.uml.actor;pointerEvents=1;dashed=0;whiteSpace=wrap;html=1;aspect=fixed;"
        vertex="1" parent="1">
  <mxGeometry x="110" y="80" width="60" height="80" as="geometry" />
</mxCell>
 
<!-- Service box -->
<mxCell id="actorB" value="API Server"
        style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff2cc;strokeColor=#d6b656;"
        vertex="1" parent="1">
  <mxGeometry x="480" y="100" width="160" height="60" as="geometry" />
</mxCell>
 
<!-- Lifeline — floating edge: uses sourcePoint/targetPoint, NOT source/target attributes -->
<mxCell id="lifA" value=""
        style="edgeStyle=none;dashed=1;endArrow=none;"
        edge="1" parent="1">
  <mxGeometry relative="1" as="geometry">
    <mxPoint x="140" y="160" as="sourcePoint" />
    <mxPoint x="140" y="700" as="targetPoint" />
  </mxGeometry>
</mxCell>
 
<!-- Activation box (thin rectangle on lifeline) -->
<mxCell id="actA1" value=""
        style="fillColor=#dae8fc;strokeColor=#6c8ebf;"
        vertex="1" parent="1">
  <mxGeometry x="130" y="220" width="20" height="180" as="geometry" />
</mxCell>
 
<!-- Synchronous message -->
<mxCell id="msg1" value="POST /orders"
        style="edgeStyle=elbowEdgeStyle;elbow=vertical;html=1;endArrow=block;endFill=1;"
        edge="1" source="actA1" target="actorB" parent="1">
  <mxGeometry relative="1" as="geometry" />
</mxCell>
 
<!-- Return message (dashed) -->
<mxCell id="msg2" value="201 Created"
        style="edgeStyle=elbowEdgeStyle;elbow=vertical;dashed=1;html=1;endArrow=open;endFill=0;"
        edge="1" source="actorB" target="actA1" parent="1">
  <mxGeometry relative="1" as="geometry" />
</mxCell>

Note: Lifelines are floating edges that use sourcePoint/targetPoint in <mxGeometry> instead of source/target attributes. This is the standard draw.io pattern for sequence diagrams.

ER Diagram

Use shape=table containers with childLayout=tableLayout. Rows are shape=tableRow cells with portConstraint=eastwest. Columns inside each row are shape=partialRectangle.

Relationship arrows use edgeStyle=entityRelationEdgeStyle:

• One-to-One: startArrow=ERone;endArrow=ERone
• One-to-Many: startArrow=ERone;endArrow=ERmany
• Many-to-Many: startArrow=ERmany;endArrow=ERmany
• Mandatory: ERmandOne, Optional: ERzeroToOne

UML Class Diagram

Class boxes are swimlane containers. Attributes and methods are plain text cells. Dividers are zero-height swimlane children.

Arrow styles by relationship type:

5. Multi-Page Diagrams

Add multiple <diagram> elements for complex systems:

<mxfile host="Electron" version="26.0.0">
  <diagram id="overview" name="Overview">
    <!-- overview mxGraphModel -->
  </diagram>
  <diagram id="detail" name="Detail View">
    <!-- detail mxGraphModel -->
  </diagram>
</mxfile>

Each page has its own independent cell id namespace. The same id value can appear in different pages without conflict.

6. Editing Existing Diagrams

When modifying an existing .drawio file:

1. Read the file first to understand existing cell ids, positions, and parent hierarchy
2. Identify the target diagram page — by index or name attribute
3. Assign new unique ids that do not collide with existing ids
4. Respect the container hierarchy — children of a swimlane use coordinates relative to their parent
5. Verify edges — after repositioning nodes, confirm edge source/target ids remain valid

Use scripts/add-shape.py to safely add a single shape without editing raw XML:

python .github/skills/draw-io-diagram-generator/scripts/add-shape.py docs/arch.drawio "New Service" 700 380

7. Best Practices

Layout

• Align shapes to the 10px grid (all coordinates divisible by 10)
• Group related shapes inside swimlane containers
• One diagram topic per page; use multi-page files for complex systems
• Aim for 40 or fewer cells per page for readability

Labels

• Add a title text cell (text;strokeColor=none;fillColor=none;fontSize=18;fontStyle=1) at top of every page
• Always set whiteSpace=wrap;html=1 on vertex shapes
• Keep labels concise — 3 words or fewer per shape where possible

Style consistency

• Use the semantic color palette from Section 3 Step 5 consistently across a project
• Prefer edgeStyle=orthogonalEdgeStyle for clean right-angle connectors
• Do not inline arbitrary HTML in labels unless necessary

File naming

• Use kebab-case: order-service-flow.drawio, database-schema.drawio
• Place diagrams alongside the code they document: docs/ or architecture/

8. Troubleshooting

9. Validation Checklist

Before delivering any generated .drawio file, verify:

• File starts with <mxfile> root element
• Every <diagram> has a non-empty id attribute
• <mxCell id="0" /> is the first cell in every diagram
• <mxCell id="1" parent="0" /> is the second cell in every diagram
• All cell id values are unique within each diagram
• Every vertex cell has vertex="1" and a child <mxGeometry as="geometry">
• Every edge cell has edge="1" and either: (a) source/target pointing to existing vertex ids, or (b) <mxPoint as="sourcePoint"> and <mxPoint as="targetPoint"> in its <mxGeometry> (floating edge — used for sequence diagram lifelines)
• Every cell (except id=0) has a parent pointing to an existing id
• html=1 is in the style for any label containing HTML tags
• XML is well-formed (no unclosed tags, no unescaped &, <, > in attribute values)
• A title label cell exists at the top of each page

Run the automated validator:

python .github/skills/draw-io-diagram-generator/scripts/validate-drawio.py <file.drawio>

10. Output Format

When delivering a diagram, always provide:

1. The .drawio file written to the requested path
2. A one-sentence summary of what the diagram shows
3. How to open it:

   "Open <filename> in VS Code — the draw.io extension will render it automatically. Or you can open it in the draw.io web app or desktop app if you prefer."

4. How to edit it (if the user is likely to customise):

   "Click any shape to select it. Double-click to edit the label. Drag to reposition."

5. Validation status — whether the validator script was run and passed

11. References

All companion files are in .github/skills/draw-io-diagram-generator/: