Concept Diagrams Skill: Flat Minimal SVG Diagrams as HTML

The Concept Diagrams skill produces clean, self-contained SVG diagrams as standalone HTML files. You describe a subject, pick a layout, and the skill generates a flat, minimal visual that renders identically in any modern browser with automatic light/dark mode. Reach for this when you need a textbook-style diagram for physics, chemistry, biology, engineering, or any domain where clarity matters more than decoration.

What it does

This skill turns a structured description into an SVG diagram wrapped in a single HTML file. The output uses a unified design system: flat colors, no gradients or shadows, consistent typography, and a fixed set of color ramps that adapt to light or dark mode automatically. You get a file you can open directly in a browser, email to a colleague, or embed in documentation. No server, no dependencies, no JavaScript runtime.

The design system enforces a specific aesthetic: minimal, educational, and consistent. Every diagram uses the same stroke widths, spacing rules, and color semantics. This makes a set of diagrams from different subjects feel like they belong to the same family.

Before you start

Prerequisites

Hermes Agent installed and running.
The skill is optional and must be installed on demand. Run:

skill_view(name="concept-diagrams", file_path="templates/template.html")

This loads the HTML template that contains the full CSS design system. The template is the foundation for every diagram you generate.

Platforms

The skill runs on Linux, macOS, and Windows.

Related skills

If your subject is software or cloud infrastructure architecture with a dark tech aesthetic, check the architecture-diagram skill first. For hand-drawn whiteboard sketches, check excalidraw. This skill is a general-purpose fallback when no more specialized skill fits.

Workflow

Decide on the diagram type from the list below.
Lay out components using the design system rules.
Write the full HTML page using templates/template.html as the wrapper. Paste your SVG where the template says .
Save as a standalone .html file (for example ~/my-diagram.html or ./my-diagram.html).
Open it directly in a browser. No server, no dependencies.

Optional: if you want a browsable gallery of multiple diagrams, see "Local Preview Server" below.

Load the HTML template:

skill_view(name="concept-diagrams", file_path="templates/template.html")

The template embeds the full CSS design system (c-* color classes, text classes, light/dark variables, arrow marker styles). The SVG you generate relies on these classes being present on the hosting page.

Design System

Philosophy

Flat: no gradients, drop shadows, blur, glow, or neon effects.
Minimal: show the essential. No decorative icons inside boxes.
Consistent: same colors, spacing, typography, and stroke widths across every diagram.
Dark-mode ready: all colors auto-adapt via CSS classes, no per-mode SVG.

Color Palette

9 color ramps, each with 7 stops. Put the class name on a <g> or shape element; the template CSS handles both modes.

Class	50 (lightest)	100	200	400	600	800	900 (darkest)
`c-purple`	#EEEDFE	#CECBF6	#AFA9EC	#7F77DD	#534AB7	#3C3489	#26215C
`c-teal`	#E1F5EE	#9FE1CB	#5DCAA5	#1D9E75	#0F6E56	#085041	#04342C
`c-coral`	#FAECE7	#F5C4B3	#F0997B	#D85A30	#993C1D	#712B13	#4A1B0C
`c-pink`	#FBEAF0	#F4C0D1	#ED93B1	#D4537E	#993556	#72243E	#4B1528
`c-gray`	#F1EFE8	#D3D1C7	#B4B2A9	#888780	#5F5E5A	#444441	#2C2C2A
`c-blue`	#E6F1FB	#B5D4F4	#85B7EB	#378ADD	#185FA5	#0C447C	#042C53
`c-green`	#EAF3DE	#C0DD97	#97C459	#639922	#3B6D11	#27500A	#173404
`c-amber`	#FAEEDA	#FAC775	#EF9F27	#BA7517	#854F0B	#633806	#412402
`c-red`	#FCEBEB	#F7C1C1	#F09595	#E24B4A	#A32D2D	#791F1F	#501313

Color Assignment Rules

Color encodes meaning, not sequence. Never cycle through colors like a rainbow.

Group nodes by category, all nodes of the same type share one color.
Use c-gray for neutral/structural nodes (start, end, generic steps, users).
Use 2-3 colors per diagram, not 6+.
Prefer c-purple, c-teal, c-coral, c-pink for general categories.
Reserve c-blue, c-green, c-amber, c-red for semantic meaning (info, success, warning, error).

Light/dark stop mapping (handled by the template CSS, just use the class):

Light mode: 50 fill + 600 stroke + 800 title / 600 subtitle
Dark mode: 800 fill + 200 stroke + 100 title / 200 subtitle

Typography

Only two font sizes. No exceptions.

Class	Size	Weight	Use
`th`	14px	500	Node titles, region labels
`ts`	12px	400	Subtitles, descriptions, arrow labels
`t`	14px	400	General text

Sentence case always. Never Title Case, never ALL CAPS.
Every <text> MUST carry a class (t, ts, or th). No unclassed text.
dominant-baseline="central" on all text inside boxes.
text-anchor="middle" for centered text in boxes.

Width estimation (approx):

14px weight 500: ~8px per character
12px weight 400: ~6.5px per character
Always verify: box_width >= (char_count × px_per_char) + 48 (24px padding each side)

Spacing & Layout

ViewBox: viewBox="0 0 680 H" where H = content height + 40px buffer.
Safe area: x=40 to x=640, y=40 to y=(H-40).
Between boxes: 60px minimum gap.
Inside boxes: 24px horizontal padding, 12px vertical padding.
Arrowhead gap: 10px between arrowhead and box edge.
Single-line box: 44px height.
Two-line box: 56px height, 18px between title and subtitle baselines.
Container padding: 20px minimum inside every container.
Max nesting: 2-3 levels deep. Deeper gets unreadable at 680px width.

Stroke & Shape

Stroke width: 0.5px on all node borders. Not 1px, not 2px.
Rect rounding: rx="8" for nodes, rx="12" for inner containers, rx="16" to rx="20" for outer containers.
Connector paths: MUST have fill="none". SVG defaults to fill: black otherwise.

Arrow Marker

Include this <defs> block at the start of every SVG:

<defs>
  <marker id="arrow" viewBox="0 0 10 10" refX="8" refY="5"
          markerWidth="6" markerHeight="6" orient="auto-start-reverse">
    <path d="M2 1L8 5L2 9" fill="none" stroke="context-stroke"
          stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
  </marker>
</defs>

Use marker-end="url(#arrow)" on lines. The arrowhead inherits the line color via context-stroke.

CSS Classes (Provided by the Template)

The template page provides:

Text: .t, .ts, .th
Neutral: .box, .arr, .leader, .node
Color ramps: .c-purple, .c-teal, .c-coral, .c-pink, .c-gray, .c-blue, .c-green, .c-amber, .c-red (all with automatic light/dark mode)

You do not need to redefine these, just apply them in your SVG. The template file contains the full CSS definitions.

SVG Boilerplate

Every SVG inside the template page starts with this exact structure:

<svg width="100%" viewBox="0 0 680 {HEIGHT}" xmlns="http://www.w3.org/2000/svg">
  <defs>
    <marker id="arrow" viewBox="0 0 10 10" refX="8" refY="5"
            markerWidth="6" markerHeight="6" orient="auto-start-reverse">
      <path d="M2 1L8 5L2 9" fill="none" stroke="context-stroke"
            stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
    </marker>
  </defs>

  <!-- Diagram content here -->

</svg>

Replace {HEIGHT} with the actual computed height (last element bottom + 40px).

Node Patterns

Single-line node (44px):

<g class="node c-blue">
  <rect x="100" y="20" width="180" height="44" rx="8" stroke-width="0.5"/>
  <text class="th" x="190" y="42" text-anchor="middle" dominant-baseline="central">Service name</text>
</g>

Two-line node (56px):

<g class="node c-teal">
  <rect x="100" y="20" width="200" height="56" rx="8" stroke-width="0.5"/>
  <text class="th" x="200" y="38" text-anchor="middle" dominant-baseline="central">Service name</text>
  <text class="ts" x="200" y="56" text-anchor="middle" dominant-baseline="central">Short description</text>
</g>

Connector (no label):

<line x1="200" y1="76" x2="200" y2="120" class="arr" marker-end="url(#arrow)"/>

Container (dashed or solid):

<g class="c-purple">
  <rect x="40" y="92" width="600" height="300" rx="16" stroke-width="0.5"/>
  <text class="th" x="66" y="116">Container label</text>
  <text class="ts" x="66" y="134">Subtitle info</text>
</g>

Diagram Types

Choose the layout that fits the subject:

Flowchart, CI/CD pipelines, request lifecycles, approval workflows, data processing. Single-direction flow (top-down or left-right). Max 4-5 nodes per row.
Structural / Containment, Cloud infrastructure nesting, system architecture with layers. Large outer containers with inner regions. Dashed rects for logical groupings.
API / Endpoint Map, REST routes, GraphQL schemas. Tree from root, branching to resource groups, each containing endpoint nodes.
Microservice Topology, Service mesh, event-driven systems. Services as nodes, arrows for communication patterns, message queues between.
Data Flow, ETL pipelines, streaming architectures. Left-to-right flow from sources through processing to sinks.
Physical / Structural, Vehicles, buildings, hardware, anatomy. Use shapes that match the physical form, <path> for curved bodies, <polygon> for tapered shapes, <ellipse>/<circle> for cylindrical parts, nested <rect> for compartments. See references/physical-shape-cookbook.md.
Infrastructure / Systems Integration, Smart cities, IoT networks, multi-domain systems. Hub-spoke layout with central platform connecting subsystems. Semantic line styles (.data-line, .power-line, .water-pipe, .road). See references/infrastructure-patterns.md.
UI / Dashboard Mockups, Admin panels, monitoring dashboards. Screen frame with nested chart/gauge/indicator elements. See references/dashboard-patterns.md.

For physical, infrastructure, and dashboard diagrams, load the matching reference file before generating, each one provides ready-made CSS classes and shape primitives.

Validation Checklist

Before finalizing any SVG, verify ALL of the following:

Every <text> has class t, ts, or th.
Every <text> inside a box has dominant-baseline="central".
Every connector <line> or <path> used as arrow has fill="none".
No arrow line crosses through an unrelated box.
box_width >= (longest_label_chars × 8) + 48 for 14px text.
box_width >= (longest_label_chars × 6.5) + 48 for 12px text.
ViewBox height = bottom-most element + 40px.
All content stays within x=40 to x=640.
Color classes (c-*) are on <g> or shape elements, never on <text> or connectors.
Arrow <defs> block is present.
No gradients, shadows, blur, or glow effects.
Stroke width is 0.5px on all node borders.

Output & Preview

Default: standalone HTML file

Write a single .html file the user can open directly. No server, no dependencies, works offline. Pattern:

# 1. Load the template
template = skill_view("concept-diagrams", "templates/template.html")

# 2. Fill in title, subtitle, and paste your SVG
html = template.replace(
    "<!-- DIAGRAM TITLE HERE -->", "SN2 reaction mechanism"
).replace(
    "<!-- OPTIONAL SUBTITLE HERE -->", "Bimolecular nucleophilic substitution"
).replace(
    "<!-- PASTE SVG HERE -->", svg_content
)

# 3. Write to a user-chosen path (or ./ by default)
write_file("./sn2-mechanism.html", html)

Tell the user how to open it:

# macOS
open ./sn2-mechanism.html
# Linux
xdg-open ./sn2-mechanism.html

Optional: local preview server (multi-diagram gallery)

Only use this when the user explicitly wants a browsable gallery of multiple diagrams.

Rules:

Bind to 127.0.0.1 only. Never 0.0.0.0. Exposing diagrams on all network interfaces is a security hazard on shared networks.
Pick a free port (do NOT hard-code one) and tell the user the chosen URL.
The server is optional and opt-in, prefer the standalone HTML file first.

Recommended pattern (lets the OS pick a free ephemeral port):

# Put each diagram in its own folder under .diagrams/
mkdir -p .diagrams/sn2-mechanism
# ...write .diagrams/sn2-mechanism/index.html...

# Serve on loopback only, free port
cd .diagrams && python3 -c "
import http.server, socketserver
with socketserver.TCPServer(('127.0.0.1', 0), http.server.SimpleHTTPRequestHandler) as s:
    print(f'Serving at http://127.0.0.1:{s.server_address[1]}/')
    s.serve_forever()
" &

If the user insists on a fixed port, use 127.0.0.1:<port>, still never 0.0.0.0. Document how to stop the server (kill %1 or pkill -f "http.server").

Examples Reference

The examples/ directory ships 15 complete, tested diagrams. Browse them for working patterns before writing a new diagram of a similar type:

File	Type	Demonstrates
`hospital-emergency-department-flow.md`	Flowchart	Priority routing with semantic colors
`feature-film-production-pipeline.md`	Flowchart	Phased workflow, horizontal sub-flows
`automated-password-reset-flow.md`	Flowchart	Auth flow with error branches
`autonomous-llm-research-agent-flow.md`	Flowchart	Loop-back arrows, decision branches
`place-order-uml-sequence.md`	Sequence	UML sequence diagram style
`commercial-aircraft-structure.md`	Physical	Paths, polygons, ellipses for realistic shapes
`wind-turbine-structure.md`	Physical cross-section	Underground/above-ground separation, color coding
`smartphone-layer-anatomy.md`	Exploded view	Alternating left/right labels, layered components
`apartment-floor-plan-conversion.md`	Floor plan	Walls, doors, proposed changes in dotted red
`banana-journey-tree-to-smoothie.md`	Narrative journey	Winding path, progressive state changes
`cpu-ooo-microarchitecture.md`	Hardware pipeline	Fan-out, memory hierarchy sidebar
`sn2-reaction-mechanism.md`	Chemistry	Molecules, curved arrows, energy profile
`smart-city-infrastructure.md`	Hub-spoke	Semantic line styles per system
`electricity-grid-flow.md`	Multi-stage flow	Voltage hierarchy, flow markers
`ml-benchmark-grouped-bar-chart.md`	Chart	Grouped bars, dual axis

Load any example with:

skill_view(name="concept-diagrams", file_path="examples/<filename>")

Quick Reference: What to Use When

User says	Diagram type	Suggested colors
"show the pipeline"	Flowchart	gray start/end, purple steps, red errors, teal deploy
"draw the data flow"	Data pipeline (left-right)	gray sources, purple processing, teal sinks
"visualize the system"	Structural (containment)	purple container, teal services, coral data
"map the endpoints"	API tree	purple root, one ramp per resource group
"show the services"	Microservice topology	gray ingress, teal services, purple bus, coral workers
"draw the aircraft/vehicle"	Physical	paths, polygons, ellipses for realistic shapes
"smart city / IoT"	Hub-spoke integration	semantic line styles per subsystem
"show the dashboard"	UI mockup	dark screen, chart colors: teal, purple, coral for alerts
"power grid / electricity"	Multi-stage flow	voltage hierarchy (HV/MV/LV line weights)
"wind turbine / turbine"	Physical cross-section	foundation + tower cutaway + nacelle color-coded
"journey of X / lifecycle"	Narrative journey	winding path, progressive state changes
"layers of X / exploded"	Exploded layer view	vertical stack, alternating labels
"CPU / pipeline"	Hardware pipeline	vertical stages, fan-out to execution ports
"floor plan / apartment"	Floor plan	walls, doors, proposed changes in dotted red
"reaction mechanism"	Chemistry	atoms, bonds, curved arrows, transition state, energy profile

When not to use it

Look elsewhere first for:

Dedicated software / cloud infrastructure architecture with a dark tech aesthetic (consider architecture-diagram if available)
Hand-drawn whiteboard sketches (consider excalidraw if available)
Animated explainers or video output (consider an animation skill)

If a more specialized skill is available for the subject, prefer that. If none fits, this skill can serve as a general-purpose SVG diagram fallback, the output will carry the clean educational aesthetic described below, which is a reasonable default for almost any subject.

Limits and gotchas

The design system is flat and minimal. Do not add gradients, drop shadows, blur, glow, or neon effects. The validation checklist enforces this.
Color encodes meaning, not sequence. Never cycle through colors like a rainbow. Use 2-3 colors per diagram, not 6+.
Only two font sizes exist: 14px weight 500 for titles, 12px weight 400 for subtitles and descriptions. No exceptions.
Every <text> must carry a class (t, ts, or th). No unclassed text.
Connector paths must have fill="none". SVG defaults to fill: black otherwise.
The local preview server must bind to 127.0.0.1 only. Never 0.0.0.0. Exposing diagrams on all network interfaces is a security hazard on shared networks.
The server is optional and opt-in. Prefer the standalone HTML file first.
Max nesting is 2-3 levels deep. Deeper gets unreadable at 680px width.
Stroke width is 0.5px on all node borders. Not 1px, not 2px.

What pairs with this

architecture-diagram for software/cloud infrastructure architecture with a dark tech aesthetic.
excalidraw for hand-drawn whiteboard sketches.

Concept Diagrams: Generate Flat, Minimal Educational SVG Visuals as HTML