svg-diagram

SVG Diagramming Conventions

Output location and referencing

Diagrams go in SVG, never ASCII art. That rule decides whether to draw; this skill covers how, starting with where the file lands.

• Put the file in an assets/ directory next to the document (e.g. docs/foo.md → docs/assets/foo-arch.svg)
• Reference it from markdown with a relative path, ![title](assets/xxx.svg). Do not inline SVG XML.
• When data in the document changes, the corresponding SVG must be updated too. A figure that contradicts the prose beside it is worse than no figure

Core principles

Get it right on the first pass

The first SVG you generate must already satisfy every item in the verification checklist at the end of this document. Do not rely on user feedback to fix basic layout problems (spacing, padding, alignment, viewBox clipping). After generating, walk the checklist yourself and fix anything you find before showing the result.

No overlapping elements

• Text must not sit on top of boxes or lines
• Lines must not cut through boxes (unless the line is an arrow terminating at that box)
• Keep 10–20px between text and lines
• For spacing between sibling/adjacent boxes see the "Block spacing" section below (single standard: ≥25px, 25–30px recommended)
• Put color-key legends outside the diagram body (bottom or side), never overlapping the content area

<!-- ✅ Correct: label sits beside the line -->
<path d="M35,200 C 35,120 ..." .../>
<text x="55" y="200">Label text</text>  <!-- 20px clearance -->

<!-- ❌ Wrong: label sits on the line -->
<text x="35" y="200">Label text</text>

Visual balance

The left and right halves should carry similar visual weight; add or remove annotation boxes to even them out.

Layout rules

Centering

content width = right edge of rightmost element - left edge of leftmost element
left margin   = (viewBox width - content width) / 2

Center the title on the content center, not the viewBox center* (they differ whenever the content is asymmetric):

<!-- content center = (content left edge + content right edge) / 2 -->
<text x="[content center x]" y="[top_padding + font_ascent]" text-anchor="middle">Title</text>

Only when the content is left-right symmetric does content center x = viewBox width / 2. When it is asymmetric, center the whole group with <g transform="translate(...)"> and update the title's x to the new content center as well.

Shifting everything after the fact

If you only notice asymmetric left/right (or top/bottom) padding after the SVG is finished, you do not need to touch every coordinate. Wrap the content in one <g transform="translate(dx, dy)">:

<svg viewBox="0 0 800 460" ...>
  <defs>...</defs>
  <g transform="translate(22, 0)">
    <!-- all content here, coordinates unchanged -->
  </g>
</svg>

Computing the offset: dx = (right padding - left padding) / 2, where padding is the distance from the viewBox edge to the nearest content element.

• dx > 0: shift content right (use when content sits too far left)
• dx < 0: shift content left (use when content sits too far right)

When to use it: the content as a whole is off-center and you do not want to edit every x coordinate. Downside: the coordinates in the file no longer equal rendered positions, so later edits require mentally subtracting the offset — if a larger layout rework is coming, fold the offset into the individual coordinates and drop the <g>.

viewBox margins

The title is part of the content, so top and bottom margins are measured from the title:

title y      = margin + font ascent (≈ font-size × 0.75)
viewBox height = bottom y of content + margin

Recommended margin: 20–25px

<!-- Example: 16px title, 20px margin -->
<!-- title y = 20 + 16×0.75 ≈ 32 -->
<!-- content bottom 270px, viewBox height = 270 + 25 = 295 -->
<svg viewBox="0 0 680 295" ...>
  <text x="340" y="32" font-size="16">Title</text>
  ...
</svg>

Box layout

• Boxes in the same row differ in size by ≤60px; text is centered (text-anchor="middle")
• Boxes in the same column share a center line
• An outer box fully encloses the inner boxes: outer size = content size + padding × 2

Dashed grouping boxes (containers)

Used to group related boxes together (e.g. "Server", "Client", "Employee instances").

Style consistency: within one diagram, every dashed grouping box must share these attributes:

• stroke-dasharray (recommended 6,4)
• rx (corner radius, recommended 8–12)
• fill (recommended #f8fafc or none)
• Inner padding (recommended 15–20px)
• Title font size (recommended 11px, to distinguish it from the 12px box body text)

Title placement: put the group title inside the top-left corner of the box (x = box x + 10, y = box y + 14) using the secondary text color #64748b.

Vertical-centering trap: compute the group box's vertical position from the combined height of title + inner content, not from the box alone. With a title, the top of the box is occupied for roughly 20–25px, and inner boxes start below it:

<!-- ✅ Group box: title + content laid out as a whole -->
<!-- box top y=100, title takes 20px, inner boxes start at y=125 -->
<rect x="50" y="100" width="200" height="120" rx="10"
      stroke="#94a3b8" stroke-dasharray="6,4" fill="#f8fafc"/>
<text x="60" y="114" font-size="11" fill="#64748b">Server</text>
<!-- inner boxes -->
<rect x="65" y="125" width="170" height="36" rx="6" .../>
<rect x="65" y="170" width="170" height="36" rx="6" .../>

Connectors pointing at a group: when a connector logically targets a set of elements rather than one of them, terminate it on the group box boundary instead of on a single member. Alternatively, wrap the elements in a grouping box and point the arrow at that box.

Box dimensions

Derive box height from the font size so there is enough inner padding:

Where the formula comes from: font-size × 3 = top padding (≈font-size) + glyph height (≈font-size) + bottom padding (≈font-size). Scale the same ratio for non-standard font sizes.

Recommended line height: font-size × 1.5 (e.g. 18px for a 12px font)

Vertically centering text in a box (baseline positioning)

In SVG, <text y=...> is the baseline, not the top of the glyph. To center text optically inside a box:

text y = box center y + font-size × 0.35
       = box y + box height/2 + font-size × 0.35

The 0.35 factor is an empirical value for "optical glyph center to baseline" (roughly one third of the font size).

Keep the two factors apart:

• font-size × 0.75 (ascent): for elements positioned from the top, such as titles (y = top_padding + ascent)
• font-size × 0.35 (baseline offset): for vertical centering inside a box

<!-- One line: height = 12 × 3 = 36px -->
<rect x="30" y="50" width="110" height="36" rx="6" .../>
<!-- y = 50 + 36/2 + 12×0.35 = 50 + 18 + 4.2 ≈ 72 -->
<text x="85" y="72" ...>Single line</text>

<!-- Two lines: height = 36 + 18 = 54px -->
<rect x="30" y="50" width="110" height="54" rx="6" .../>
<text x="85" y="70" ...>First line</text>
<text x="85" y="88" ...>Second line</text>  <!-- 18px apart -->

Display size

<svg viewBox="0 0 750 400" width="750" xmlns="http://www.w3.org/2000/svg">

Connector rules

Choosing a connector

Use a straight L line for co-axial connections; use smooth C/Q curves whenever the path turns or routes around something. Never use right-angle elbows.

<!-- ✅ Horizontally co-axial: straight line -->
<path d="M190,77 L 320,77" .../>

<!-- ✅ Vertically co-axial: straight line -->
<path d="M130,145 L 130,213" .../>

<!-- ❌ Right-angle elbow for a turn -->
<path d="M100,100 L100,200 L200,200" .../>

<!-- ✅ Smooth curve for a turn -->
<path d="M100,100 C 100,150 150,200 200,200" .../>

Arrow rules

• Direction comes from the final path segment: rightward means x increases, downward means y increases
• Symmetric clearance: 5px between the arrow and the source box, and the same visual clearance at the target box
• Centered labels: x = (source right edge + target left edge) / 2, together with text-anchor="middle"

Fixing arrow direction on curved paths: orient="auto" aligns the arrowhead with the tangent at the end of the path. The end tangent of a C/Q curve can be skewed, which makes the arrowhead look crooked. The fix: place the second control point (cp2) so that the direction cp2 → end point is exactly the direction you want the arrowhead to face.

<!-- ❌ cp2 placed arbitrarily, arrow direction uncontrolled -->
<path d="M100,100 C 100,200 250,200 300,250" marker-end="url(#arrow)"/>

<!-- ✅ cp2 shares x with the end point, tangent naturally points down -->
<path d="M100,100 C 100,200 300,220 300,250" marker-end="url(#arrow)"/>

<!-- ✅ cp2 shares y with the end point, tangent naturally points right -->
<path d="M100,100 C 100,200 270,250 300,250" marker-end="url(#arrow)"/>

Rules for controlling arrow direction:

Computing path endpoints (5px clearance, arrowhead extends 6px past the end of the line):

Core rule: start 5px away from the source box, end 11px away from the target box (5px clearance + 6px arrowhead extension).

Because we use refX="2" (the notch at the tail of the arrowhead), the tip extends 6px (8-2=6) beyond the end of the line, and the line joins the arrowhead at the center of its notch, so the join reads as continuous.

Formulas for the four directions:

<!-- Rightward arrow →: source right edge 180, target left edge 220 -->
<!-- start: 180+5=185, end: 220-11=209, tip reaches 209+6=215 -->
<path d="M185,70 L 209,70" marker-end="url(#arrow)"/>

<!-- Leftward arrow ←: source left edge 220, target right edge 180 -->
<!-- start: 220-5=215, end: 180+11=191, tip reaches 191-6=185 -->
<path d="M215,70 L 191,70" marker-end="url(#arrow)"/>

<!-- Downward arrow ↓: source bottom 140, target top 170 -->
<!-- start: 140+5=145, end: 170-11=159, tip reaches 159+6=165 -->
<path d="M130,145 L 130,159" marker-end="url(#arrow)"/>

<!-- Upward arrow ↑: source top 170, target bottom 140 -->
<!-- start: 170-5=165, end: 140+11=151, tip reaches 151-6=145 -->
<path d="M130,165 L 130,151" marker-end="url(#arrow)"/>

Bidirectional example (two boxes with arrows going both up and down):

<!-- box A bottom=375, box B top=420 -->
<!-- downward arrow: A→B -->
<path d="M320,380 L 320,409" marker-end="url(#arrow)"/>  <!-- 375+5, 420-11 -->
<!-- upward arrow: B→A -->
<path d="M480,415 L 480,386" marker-end="url(#arrow)"/>  <!-- 420-5, 375+11 -->

Routing around obstacles

Keep detour paths at least 20px outside the obstacle's boundary:

<!-- Gateway right edge=620, detour path runs at x=650 -->
<path d="M600,313 Q 650,313 650,400 Q 650,640 635,685"
      fill="none" stroke="#a855f7" stroke-dasharray="6,4" marker-end="url(#arrow)"/>

Block spacing (single global standard)

Spacing between adjacent boxes/blocks is ≥25px, 25–30px recommended.

Breakdown: 5px start clearance + 11px end clearance and arrowhead extension + ≥6px of visible line + safety margin ≈ 25px. At least 6px of visible line, otherwise the arrow degenerates into a dot.

Stay compact: do not exceed 30px. Spacing that is too wide (>40px) makes the whole diagram feel loose and the connectors overly long. Aim for 5px / 11px at the two ends and 6–12px of visible line in between.

Overall density: when you are done, step back and judge the density — if the content area is clearly small relative to the viewBox (large blank regions), tighten spacing or shrink the viewBox. Common causes: box spacing >30px, viewBox margin >25px, excessive padding inside grouping boxes.

This is the only block-spacing standard in this document; both "No overlapping elements" and the verification checklist refer back to it.

SVG paint order (z-order)

SVG has no z-index — elements painted later sit on top. Order them like this:

<!-- 1. Bottom layer: background boxes (dashed grouping boxes, swimlanes) -->
<rect ... stroke-dasharray="6,4" fill="#f8fafc"/>

<!-- 2. Middle layer: connectors and arrows -->
<path d="..." marker-end="url(#arrow)"/>

<!-- 3. Top layer: boxes and text -->
<rect ... fill="#dbeafe"/>
<text ...>Label</text>

Common mistake: painting connectors first and the dashed background box afterwards → the connectors get covered.

Exception: cross-layer loop-back lines: connectors that must span several boxes — iteration loops, cross-region dashed lines — have to be painted after the boxes (layer 4), otherwise the boxes hide them:

<!-- 1. background boxes -->
<!-- 2. ordinary connectors -->
<!-- 3. boxes and text -->
<!-- 4. cross-layer loop-back lines (topmost, painted over the boxes) -->
<path d="..." stroke-dasharray="6,4" marker-end="url(#arrow-purple)"/>

Estimating text width (overflow protection)

Before placing text, estimate its right edge and confirm it does not intrude on neighboring elements:

Estimation formulas:

• text-anchor="start": right edge ≈ x + char count × char width
• text-anchor="middle": left/right edge ≈ x ± (char count × char width) / 2
• text-anchor="end": left edge ≈ x - char count × char width

Check before placing: text right edge + 10px < left edge of the neighbor to its right.

<!-- ❌ Text "4K tokens (= mini-batch)" starts at x=274 -->
<!-- 24 chars × 5px ≈ 120px, right edge ≈ 394, intrudes on the box at x=350 -->
<text x="274" y="115" font-size="9">4K tokens (= mini-batch)</text>

<!-- ✅ Moved below the box and centered, intrudes on nothing -->
<text x="154" y="138" font-size="9" text-anchor="middle">4K tokens (= mini-batch)</text>

Placing labels on curves

A label describing an arc or curve must not sit on the path itself (the line would run through the text) — put it on the convex or concave side of the curve:

<!-- ❌ Label at y=218 while the curve passes near y=225: the line cuts the text -->
<text x="350" y="218">Skip Connection</text>
<path d="M82,165 C 82,225 620,225 646,225" .../>

<!-- ✅ Label at y=222, lowest point of the curve at y=260, 38px apart -->
<text x="350" y="222">Skip Connection</text>
<path d="M82,149 C 82,260 600,255 640,255" .../>

Rule: keep ≥15px between the label and the nearest point on the curve. For a downward-bending curve put the label above it; for an upward-bending curve put it below.

Coordinate management

Record boundaries in comments

<!-- Gateway: x=280, y=55, width=340, height=435, right edge=620, bottom edge=490 -->
<rect x="280" y="55" width="340" height="435" .../>

When you move an element, update in lockstep

1. viewBox dimensions
2. Every arrow to/from that element (start point, end point, control points)
3. Positions of related labels
4. The dashed background/grouping box that wraps the element (swimlane, container) — the most commonly missed one
5. Other boxes aligned with it in the same row/column
6. Any other affected elements

Style rules

Fonts (required)

Every SVG must declare a CJK-capable font stack in `<style>` — this is non-negotiable:

<style>
  text { font-family: 'PingFang SC', 'Microsoft YaHei', 'Noto Sans CJK SC', system-ui, sans-serif; }
</style>

Fallback order: macOS (PingFang SC) → Windows (Microsoft YaHei) → Linux (Noto Sans CJK SC) → system default. `Noto Sans CJK SC` must be kept, otherwise server-side rendering on Linux falls back to a font with no CJK coverage.

Vertical text: <text writing-mode="tb">vertical text</text>

Colors

Base colors: primary text #1e293b, secondary text #64748b, muted text / arrows #94a3b8

Semantic colors:

Arrowhead definitions

Use a notched arrowhead rather than a plain triangle — it reads better:

<defs>
  <!-- Default gray arrow (markerUnits="userSpaceOnUse" is mandatory) -->
  <marker id="arrow" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#64748b"/>
  </marker>
  <!-- Semantic-color arrows (matching the box stroke colors) -->
  <marker id="arrow-blue" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#3b82f6"/>
  </marker>
  <marker id="arrow-orange" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#f59e0b"/>
  </marker>
  <marker id="arrow-green" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#22c55e"/>
  </marker>
  <marker id="arrow-purple" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#a855f7"/>
  </marker>
  <marker id="arrow-red" markerWidth="8" markerHeight="8" refX="2" refY="4" orient="auto" markerUnits="userSpaceOnUse">
    <path d="M0,0 L8,4 L0,8 L2,4 z" fill="#ef4444"/>
  </marker>
</defs>

Arrow color reference:

Key parameters:

• orient="auto" rotates the arrowhead to follow the path direction
• markerUnits="userSpaceOnUse" must be declared explicitly, otherwise the default strokeWidth scales the arrowhead with line thickness (at stroke-width=2 the arrow doubles in size and blows past the intended clearance)
• refX="2" aligns the end of the line with the center of the arrowhead's tail notch, so the join looks natural
• refY="4" centers it vertically (arrow height 8, midpoint 4)
• The arrow path M0,0 L8,4 L0,8 L2,4 z forms the notched shape, with the tip at x=8
• The tip extends 6px (8-2=6) beyond the end of the line, so the end point is computed as target edge - 11

Arrows on thick lines: when stroke-width > 1.5 (e.g. heavy dashed lines), an 8×8 arrowhead looks too small. Define a proportionally larger marker:

<!-- Large arrow for thick lines (1.5×): tip extends 12-3=9px -->
<marker id="arrow-red-lg" markerWidth="12" markerHeight="12" refX="3" refY="6"
        orient="auto" markerUnits="userSpaceOnUse">
  <path d="M0,0 L12,6 L0,12 L3,6 z" fill="#ef4444"/>
</marker>

XML special characters (high risk, always check)

SVG is XML, so special characters in text must be escaped or the entire SVG fails to render:

<!-- ❌ Wrong: unescaped & -->
<text>Load & Save</text>

<!-- ✅ Correct: escaped -->
<text>Load &amp; Save</text>

Escaping when generating SVG programmatically

When generating SVG from Python/JS, text coming from external sources (CSV, database, API) must be escaped before it is concatenated into the SVG:

def svg_escape(text):
    return str(text).replace('&','&amp;').replace('<','&lt;').replace('>','&gt;').replace('"','&quot;')

# ❌ Splicing external data directly ("Research&Development" → XML parse failure, image won't render)
svg += f'<text>{team_name}</text>'

# ✅ Escape first
svg += f'<text>{svg_escape(team_name)}</text>'

High-risk data sources: team names (contain &), requirement descriptions (contain <>), user input, file names

Troubleshooting

Verification checklist

• [ ] No overlapping elements (text, boxes, lines)
• [ ] No text right edge intrudes on a neighbor (estimated from character widths)
• [ ] ≥15px between a curve label and the nearest point on the curve
• [ ] Symmetric arrow clearance at both ends (5px), visible line segment ≥6px
• [ ] Markers declare markerUnits="userSpaceOnUse"
• [ ] Thick lines (stroke-width > 1.5) use the enlarged marker
• [ ] Connectors use C/Q curves, no right angles
• [ ] Labels centered (text-anchor="middle")
• [ ] Detour paths stay outside obstacles
• [ ] Block spacing ≥25px (25–30px recommended, see "Block spacing")
• [ ] Title centered, boxes in a row similar in size
• [ ] width attribute present, viewBox matches the content
• [ ] Box heights sufficient (one line ≥ font-size × 3)
• [ ] Top and bottom viewBox margins comparable (20–25px, measured from the top of the title)
• [ ] Left and right viewBox padding symmetric (use <g transform="translate(dx,0)"> to shift if not)
• [ ] Special characters escaped (& → &, < → <)
• [ ] Every element inside the visible viewBox (bottom = bottom edge of the lowest element + 25px)
• [ ] Every connector has a clear meaning (labeled, or its source and target are inferable from context)
• [ ] Dashed grouping boxes styled consistently (dasharray, corner radius, fill, padding, title font size)
• [ ] No large blank regions (spacing within 30px, viewBox hugs the content)

Editing workflow

1. Collect: gather the full set before touching anything

When you receive a request to modify an SVG, do not start editing immediately. First judge:

• The user gave one change → reply "anything else you want adjusted?" to get the full list in one go
• The user gave several changes → move to execution and apply them all at once

Goal: fewer round trips, so you are not re-emitting the whole SVG for every single tweak.

2. Execute: apply everything at once, with a change list

Once all edits are done, print the change list first, then show the final SVG:

Change list:
1. Title spacing 20px → 30px
2. Box A fill #dbeafe → #f3e8ff
3. Connector L1 path adjusted (routes around the new box)

Do not show the diagram after each individual edit — the user only needs the final result.

3. Layout tweaks: understand the intent and adjust dependents

When the user says "move box A 20px to the right", do not mechanically change box A's x alone. Handle the dependents too:

• Every arrow path to/from box A
• Alignment with the other boxes in box A's row
• The position of box A's text labels
• The size of the outer/background box wrapping box A

Spacing/alignment requests ("make it tighter", "left-align it", "center it") → read the overall intent and adjust all related elements in one pass, so the user does not have to iterate pixel by pixel.