diagrams-drawio/references/rules-style.md

diagrams-drawio — Style Rules

Visual appearance and sizing rules for draw.io diagrams generated by this skill.
These rules are MANDATORY — apply them when generating any diagram.

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General style rules — MANDATORY

• All child shapes within the same swimlane must have the same height — never mix different heights within one swimlane layer/row
• Rectangles must never use rounding — always set rounded=0 on rectangle shapes
• Prefer dimensions divisible by 40; wrap non-divisible vendor icons in a grid-aligned container
• The space between sibling shapes of the same level (same swimlane, same row) must be divisible by 40px — use 40px as the minimum gap. If a swimlane row has shapes of width W and gap G, the step between shape origins = W + G where G ≥ 40 and G mod 40 = 0.
• Swimlane startSize (header height) must be divisible by 40 — use startSize=40
• Swimlane body height (total − startSize) must be divisible by 40 — so total height = 40 + N×40 = a multiple of 40
• Example: startSize=40, body=80 → total=120 ✓; body=160 → total=200 ✓
• Use ≤ 3 primary color families; create hierarchy with shades
• Use dashed connectors only for semantically distinct flows (async, backup, admin)
• Shape size is driven by content, not routing — typically 80–160px tall per label line

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Connector routing rules — MANDATORY

• All outgoing connectors from the same shape must share a single exit point — never let multiple connectors leave a shape at different X/Y coordinates. Pick one exit side (top/bottom/left/right) and one coordinate on that side; all outgoing connectors use it as their first waypoint.
• All incoming connectors to the same shape must share a single entry point — never let multiple connectors enter a shape at different X/Y coordinates. Pick one entry side and one coordinate; all incoming connectors use it as their last waypoint before the shape.
• Connectors must never attach at a shape corner — the exit/entry point must lie on the middle of a side: top-center, bottom-center, left-center, or right-center. A point that is simultaneously on both an X-edge (left or right) AND a Y-edge (top or bottom) of the shape bounding box is a corner and is forbidden. Use the midpoint of the chosen side: x_mid = (x1+x2)/2 for top/bottom sides, y_mid = (y1+y2)/2 for left/right sides.
• These rules ensure a clean "bus-bar" fan-out/fan-in pattern and prevent connectors from diverging at their source or converging at their target at different positions, which creates visual clutter and increases crossing risk.
• Exception: shapes with only a single outgoing or single incoming connector — the single-port rule trivially holds. The corner rule still applies.
• When computing waypoints: use page-negative-space-summary to find free X corridors per row, then pick an exit/entry coordinate that lies within a free corridor at the next traversed level. This minimises connector-shape overlaps and connector crossings.
• Waypoints must never be closer than 20px to any shape — the first waypoint after a shape exit must be at least 20px away from the shape's edge in the direction of travel (e.g., if exiting bottom at y=520, first waypoint y ≥ 540; if exiting right at x=1040, first waypoint x ≥ 1060). The last waypoint before a shape entry must likewise be at least 20px away from the shape's edge. This 20px clearance also applies to any waypoint relative to same-level sibling shapes the connector passes by — the waypoint must not come within 20px of any sibling shape's bounding box side it is adjacent to.

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Text-width estimation — negative space calculation

When computing negative space manually (e.g. for a negative-space diagram), shape bounding boxes alone overestimate occupied space. Text labels only occupy a sub-region within the shape bbox. Use the following formula to estimate the text rendering width:

charWidth  = fontSize × 0.6           // avg glyph width for proportional fonts
rawWidth   = longestLineChrCount × charWidth
padding    = fontSize × 1.0           // horizontal padding (~0.5em each side)
textWidth  = rawWidth + padding

fontStyle modifiers (draw.io flags):
  bit 0 (value 1) = bold   → charWidth × 1.10
  bit 1 (value 2) = italic → charWidth × 1.05

textXMin = clamp(shapeCenterX − textWidth/2, shape.xMin, shape.xMax)
textXMax = clamp(shapeCenterX + textWidth/2, shape.xMin, shape.xMax)

When to apply:

• Swimlane headers (startSize=40 band): the header text is centered in the full zone width. Use text region for occupied X, flanks are free negative space.
• Large container shapes whose children don't fill the full width.
• Any shape where textWidth < shapeWidth — the flanks inside the bbox are free.

Example — zone headers at fontSize=12 bold (fontStyle=1), center x=660, zone x=80..1240:

Label	chars	charWidth	rawWidth	padding	textWidth	textXMin	textXMax	flank each side
"Client Layer"	12	7.9	95	12	107	607	713	~527pt
"API Gateway / Edge Layer"	24	7.9	190	12	202	559	761	~479pt
"Core Microservices"	18	7.9	143	12	155	583	737	~503pt
"Messaging / Event Bus"	21	7.9	166	12	178	571	749	~491pt
"Data Layer"	10	7.9	79	12	91	615	705	~535pt

The page-negative-space-summary action now outputs these fields per shape:

• textXMin, textXMax, textWidth — estimated text rendering region
• textFlankLeft, textFlankRight — free space flanking text inside bbox
• rows[].textAwareFreeCorridors — free corridors computed using text regions (wider than bbox-based corridors)