Excalidraw Diagram Generator Skill

Excalidraw Diagram Generator

Create Excalidraw-compatible JSON diagrams. Output .excalidraw.json files.

JSON Schema

{
  "type": "excalidraw",
  "version": 2,
  "source": "https://excalidraw.com",
  "elements": [],
  "appState": {
    "gridSize": null,
    "viewBackgroundColor": "#ffffff"
  },
  "files": {}
}

Coordinate System

Origin: Top-left corner (0, 0)
X-axis: Increases rightward
Y-axis: Increases downward
Unit: Pixels

Grid-Based Layout System

Use a 20px grid for alignment. Standard positions:

GRID_UNIT = 20
COLUMN_WIDTH = 200    # Element + gap
ROW_HEIGHT = 150      # Element + gap

Position formula:
  x = START_X + (column * COLUMN_WIDTH)
  y = START_Y + (row * ROW_HEIGHT)

Recommended START_X = 50, START_Y = 100

Standard Element Sizes

| Element | Width | Height | Use Case | |---------|-------|--------|----------| | Small box | 120 | 60 | Labels, simple steps | | Standard box | 160 | 80 | Process steps | | Large box | 200 | 100 | Detailed nodes | | Wide box | 240 | 80 | Long text | | Diamond | 100 | 100 | Decision points | | Circle | 80 | 80 | Start/End nodes | | Icon circle | 90 | 80 | With emoji |

Text Centering Formula

CRITICAL: To center text inside a shape:

text_x = shape_x + (shape_width - text_width) / 2
text_y = shape_y + (shape_height - text_height) / 2

Text Size Reference

| Font Size | Char Width | Line Height | Use | |-----------|------------|-------------|-----| | 11 | ~6px | 15px | Labels, annotations | | 12 | ~7px | 16px | Secondary text | | 14 | ~8px | 18px | Body text | | 16 | ~9px | 20px | Primary text | | 18 | ~10px | 24px | Headings | | 20 | ~11px | 26px | Titles | | 28 | ~16px | 35px | Icons/Emoji |

Estimate text_width: char_count * char_width Estimate text_height: line_count * line_height

Centering Example

Shape: x=100, y=100, width=160, height=80
Text: "Process" (7 chars), fontSize=16

text_width = 7 * 9 = 63
text_height = 1 * 20 = 20
text_x = 100 + (160 - 63) / 2 = 148.5 ≈ 148
text_y = 100 + (80 - 20) / 2 = 130

Element Types

Common Properties (required for all)

{
  "id": "unique-id",
  "type": "rectangle",
  "x": 100,
  "y": 100,
  "width": 160,
  "height": 80,
  "strokeColor": "#1e1e1e",
  "backgroundColor": "transparent",
  "fillStyle": "solid",
  "strokeWidth": 2,
  "roughness": 1,
  "opacity": 100,
  "angle": 0,
  "seed": 100,
  "version": 1,
  "isDeleted": false,
  "boundElements": null,
  "link": null,
  "locked": false
}

Type-Specific Properties

Rectangle: Add "roundness": {"type": 3} for rounded corners

Text:

{
  "type": "text",
  "text": "Content\nLine 2",
  "fontSize": 16,
  "fontFamily": 1,
  "textAlign": "center",
  "verticalAlign": "middle"
}

fontFamily: 1=Virgil (hand), 2=Helvetica, 3=Cascadia (mono)

Arrow/Line:

{
  "type": "arrow",
  "points": [[0, 0], [100, 0]],
  "startArrowhead": null,
  "endArrowhead": "arrow"
}

Points are relative to element's x,y
Arrowheads: null, "arrow", "bar", "dot", "triangle"

Arrow Positioning

Horizontal Arrow (Left to Right)

From shape at (x1, y1, w1, h1) to shape at (x2, y2, w2, h2):

arrow_x = x1 + w1           # Right edge of source
arrow_y = y1 + h1/2         # Vertical center
gap = x2 - (x1 + w1)        # Space between shapes
points = [[0, 0], [gap, 0]]

Vertical Arrow (Top to Bottom)

arrow_x = x1 + w1/2         # Horizontal center
arrow_y = y1 + h1           # Bottom edge of source
gap = y2 - (y1 + h1)
points = [[0, 0], [0, gap]]

Diagonal/Curved Arrow

points = [[0, 0], [dx/2, dy], [dx, dy]]  # Curved path

Color Palette

| Color | Stroke | Background | Use | |-------|--------|------------|-----| | Blue | #1971c2 | #a5d8ff | Primary, info | | Green | #2f9e44 | #b2f2bb | Success, start | | Orange | #e8590c | #ffc9c9 | Warning, action | | Red | #e03131 | #ffc9c9 | Error, end | | Purple | #9c36b5 | #eebefa | Special, loop | | Yellow | #f08c00 | #ffec99 | Highlight | | Teal | #099268 | #96f2d7 | Secondary | | Gray | #868e96 | #dee2e6 | Neutral, labels | | Black | #1e1e1e | - | Text, borders |

Z-Order (Element Array Order)

Elements render in array order. First = back, last = front.

Correct order:

Background shapes (containers, frames)
Connection lines/arrows
Foreground shapes (nodes, boxes)
Text labels
Icons/overlays

Layout Patterns

Horizontal Flow (Left to Right)

COL_GAP = 60  # Gap between elements

Element 1: x=50,  y=100, w=160, h=80
Arrow 1:   x=210, y=140, points=[[0,0],[60,0]]
Element 2: x=270, y=100, w=160, h=80
Arrow 2:   x=430, y=140, points=[[0,0],[60,0]]
Element 3: x=490, y=100, w=160, h=80

Vertical Flow (Top to Bottom)

ROW_GAP = 50

Element 1: x=100, y=50,  w=160, h=80
Arrow 1:   x=180, y=130, points=[[0,0],[0,50]]
Element 2: x=100, y=180, w=160, h=80

Decision Branch (Diamond)

Diamond:    x=300, y=200, w=100, h=100
Center:     (350, 250)

Yes path (up-right):
  arrow_x=350, arrow_y=200
  points=[[0,0], [0,-50], [100,-50]]

No path (down-right):
  arrow_x=350, arrow_y=300
  points=[[0,0], [0,50], [100,50]]

Feedback Loop (Return Arrow)

From bottom-right back to left:
  Start: (800, 400)
  points=[
    [0, 0],        # Start
    [0, 80],       # Down
    [-400, 80],    # Left
    [-400, -150]   # Up to target
  ]

Complete Node Example

[
  {
    "id": "node1-box",
    "type": "rectangle",
    "x": 100, "y": 100, "width": 160, "height": 80,
    "strokeColor": "#1971c2",
    "backgroundColor": "#a5d8ff",
    "fillStyle": "solid",
    "strokeWidth": 2,
    "roughness": 1,
    "opacity": 100,
    "angle": 0,
    "seed": 100,
    "version": 1,
    "isDeleted": false,
    "boundElements": [{"id": "arrow1", "type": "arrow"}],
    "roundness": {"type": 3},
    "link": null,
    "locked": false
  },
  {
    "id": "node1-text",
    "type": "text",
    "x": 130, "y": 130,
    "width": 100, "height": 20,
    "text": "Process Step",
    "fontSize": 16,
    "fontFamily": 1,
    "textAlign": "center",
    "verticalAlign": "middle",
    "strokeColor": "#1e1e1e",
    "backgroundColor": "transparent",
    "fillStyle": "solid",
    "strokeWidth": 1,
    "roughness": 1,
    "opacity": 100,
    "angle": 0,
    "seed": 101,
    "version": 1,
    "isDeleted": false,
    "boundElements": null,
    "link": null,
    "locked": false
  }
]

Positioning Checklist

Before generating:

☐ Define grid: START_X, START_Y, COLUMN_WIDTH, ROW_HEIGHT
☐ List all elements with row/column positions
☐ Calculate exact x,y using formulas
☐ Calculate text positions using centering formula
☐ Calculate arrow start points and relative endpoints
☐ Order elements correctly for z-order
☐ Assign unique IDs and incrementing seeds

Templates

Basic flowchart: assets/flowchart-template.excalidraw.json
Processing loop: references/processing-loop-template.md
Element specs: references/elements.md

Output

Save as .excalidraw.json
Copy to /mnt/user-data/outputs/
Tell user: "Open at excalidraw.com → Menu → Open"

Agent Skills: Excalidraw Diagram Generator

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Skill Files