Write YAML, get Highcharts. Efficient assembly of dashboards on well-grounded rails.

DSL Documentation · Demo Notebook · GitHub

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Trendence is a Berlin-based HR data and analytics company. We believe data visualization should be modern, AI-ready, and accessible. That's why we've been building TREVL -- the TREndence Visualization Language -- a custom DSL designed to make chart creation as simple as writing a few lines of YAML. A language humans easily can use and robots love.

AI-First Design

TREVL is built for humans and machines. Every feature is designed so that LLMs can generate, validate, and iterate on visualizations autonomously:

• Trevl.schema_reference -- compact reference (~1500 tokens) optimized for system prompts
• Trevl.validate(yaml) -- structured error feedback for self-correction loops
• Trevl.examples -- 10 annotated examples for few-shot learning
• Trevl::DataSource.for("name").field_names("endpoint") -- discover available data fields
• llms.txt -- machine-readable reference in the repo root
• JSON Schema -- formal validation for editors, CI, and AI agents

# 1. Discover: what fields does this endpoint return?
Trevl::DataSource.for("myapi").field_names("salary")
# => ["q10", "q50", "q90"]

# 2. Reference: get the compact TREVL spec for a system prompt
Trevl.schema_reference
# => "# TREVL — TREndence Visualization Language\n..."

# 3. Examples: few-shot learning material
Trevl.examples.first
# => {name: "bar_chart", description: "Simple bar chart...", yaml: "..."}

# 4. Validate: catch errors before rendering
Trevl.validate(yaml).errors
# => ["[my_chart] Missing required field(s): api, highchartsData (at )"]

# 5. Render: YAML → Highcharts JSON
Trevl.render(yaml)

Features

• Declarative YAML -- define charts, scores, tables, and filters without writing JavaScript
• Pluggable data sources -- REST APIs, CubeJS, static/in-memory data, or build your own
• Computed fields -- per-row JavaScript transformations via ExecJS
• Postprocess -- full-dataset transforms (sort, filter, aggregate) in JavaScript
• Template inheritance -- share chart styles with deep merge
• JSON Schema validation -- catch errors before rendering
• iRuby notebooks -- render interactive Highcharts directly in Jupyter
• Fully offline -- bundled Highcharts JS, no CDN needed
• Standalone -- no Rails, no framework dependencies

Installation

gem "trevl"

Prerequisites: Ruby >= 3.1, Node.js (brew install node) for computed fields.

Quick Start

require "trevl"

# 1. Register a data source
Trevl::DataSource.register("demo", Trevl::DataSource::Static.new(
  data: {
    "pirates" => {
      "data" => [
        {"name" => "Guybrush",  "barrels" => 12},
        {"name" => "LeChuck",   "barrels" => 47},
        {"name" => "Elaine",    "barrels" => 3},
        {"name" => "Stan",      "barrels" => 28},
        {"name" => "Murray",    "barrels" => 0}
      ]
    }
  }
))

# 2. Render
results = Trevl.render(<<~YAML)
  components:
  - id: rum_consumption
    type: chart
    api: demo
    highchartsData:
      chart:
        type: bar
      title:
        text: Annual Rum Consumption by Pirate (Barrels)
      colors: ["#8B4513"]
      series:
      - name: Rum (barrels)
        data:
          x: "$pirates.data.name"
          y: "$pirates.data.barrels"
YAML

# 3. Done -- results.first["highchartsData"] is ready for Highcharts

Examples Gallery

See examples/ for the YAML source of each chart.

HTML Export

Render TREVL to an HTML file:

html = Trevl.render_to_html(yaml, width: 1000, height: 500)
File.write("chart.html", html)

The output is a complete HTML document that pulls Highcharts from the CDN. Point highcharts_path at a local copy (see Highcharts) and it is embedded inline instead, which makes the file self-contained and usable offline. Open it in any browser, or take a screenshot for AI agents:

# Grover gem (Puppeteer wrapper)
Grover.new(html).to_png

# Ferrum (Chrome DevTools Protocol)
browser = Ferrum::Browser.new
browser.content = html
browser.screenshot(path: "chart.png")

# Playwright MCP (for AI agents)
# browser_navigate → browser_take_screenshot

iRuby Notebooks

require "trevl"
require "trevl/notebook"

nb = Trevl::Notebook.new

nb.chart(<<~YAML, data: {"salary" => {"data" => [...]}})
  components:
  - id: chart
    type: chart
    api: static
    highchartsData:
      chart:
        type: column
      series:
      - data:
          x: "$salary.data.level"
          y: "$salary.data.value"
YAML

Highcharts is bundled -- charts render offline. See notebooks/demo.ipynb for 5 working examples.

Data Sources

Static (in-memory)

Trevl::DataSource.register("mydata", Trevl::DataSource::Static.new(
  data: {"endpoint" => {"data" => [...], "meta" => {...}}}
))

REST API

Trevl::DataSource.register("myapi", Trevl::DataSource::Api.new(
  base_url: "https://api.example.com/v1",
  auth: Trevl::Auth::BearerToken.new(ENV["API_TOKEN"])
))

CubeJS

Trevl::DataSource.register("cube", Trevl::DataSource::Cube.new(
  url: "https://cube.example.com/cubejs-api/v1",
  token: ENV["CUBE_TOKEN"]
))

Custom

class MySource < Trevl::DataSource::Base
  def fetch(endpoint, params = {}, resource: nil)
    {"data" => MyDB.query(endpoint, params), "meta" => {}}
  end
end

Trevl::DataSource.register("db", MySource.new)

Per-render injection

Instead of registering globally, pass data directly to a render call. Per-render data takes precedence over the registry and never touches shared state — the right choice when the data differs per request (e.g. web apps serving concurrent users).

For inline rows, pass the raw hash as data: — it answers any api: name in the document, and components may omit api: entirely:

Trevl.render(yaml, data: {"scores" => rows_for_this_request})
Trevl.render_to_html(yaml, data: {"scores" => rows_for_this_request})

For full control (multiple sources, API/Cube instances), pass data_sources: — entries win over data: for their name:

source = Trevl::DataSource::Api.new(base_url: "https://api.example.com/v1")

Trevl.render(yaml, data_sources: {"mydata" => source})

Validation

Validate TREVL YAML before rendering — catch errors early, not at render time.

result = Trevl.validate(<<~YAML)
  components:
  - id: my_chart
    type: chart
YAML

result.valid?   # => false
result.errors   # => ["[my_chart] Missing required field(s): api, highchartsData (at )"]

Powered by JSON Schema (draft 2020-12) -- covers all 5 component types with conditional validation. The schema files work standalone in VS Code, CI pipelines, or any JSON Schema-compatible tool.

Ideal for AI agents: generate TREVL, validate, self-correct, render.

Full docs: trevl.trendence.com/validation

YAML Reference

Component Types

Type	Description
chart	Highcharts visualization (bar, column, line, pie, ...)
score	Single KPI value with optional unit
table	Data table with column definitions
text	Static text / HTML content
filter	Filter options bound to data

Variable References

"$endpoint.data.fieldName"             # data row field
"$endpoint.meta.fieldName"             # metadata field
"$resource.endpoint.data.fieldName"    # with resource qualifier
"$computedFieldName"                   # computed field shorthand

Computed Fields

Per-row JavaScript expressions:

computed:
- name: color
  arguments:
    val: "$salary.data.value"
  code: 'val > 60000 ? "#003F85" : "#ccc"'

Postprocess

Full-dataset JavaScript transforms:

postprocess: |
  $result = $result
    .sort((a, b) => b.value - a.value)
    .slice(0, 10);

Templates

Trevl.template_store.register("blue_bar", {
  "highchartsData" => {
    "chart" => {"type" => "bar"},
    "colors" => ["#003F85"]
  }
})

- id: my_chart
  template: blue_bar
  highchartsData:
    title:
      text: My Chart

Deep merge: component overrides template at the same path.

Auth

# Bearer token
auth = Trevl::Auth::BearerToken.new("token")

# Custom -- any object with #apply(headers, url:)
class MyAuth
  def apply(headers, url: nil)
    headers["Authorization"] = "Bearer #{fetch_token}"
  end
end

Configuration

Trevl.configure do |c|
  c.logger = Logger.new($stdout, level: :info)
  c.template_store = my_custom_store
end

Highcharts

TREVL produces Highcharts configuration; it does not ship Highcharts. Highcharts is commercial software by Highsoft and is deliberately not bundled here, so using it requires your own licence.

Trevl.render returns plain configuration hashes and never touches Highcharts at all. Only the HTML export and the notebook display load it, and by default they reference the CDN:

Trevl.configure do |c|
  c.highcharts_url = "https://code.highcharts.com/11.4.0/highcharts.js"  # default
  c.highcharts_modules = ["https://code.highcharts.com/11.4.0/highcharts-more.js"]
end

Set highcharts_path to a local file and it gets inlined instead of linked, which is what you want for offline use or air-gapped rendering:

Trevl.configure do |c|
  c.highcharts_path = "/opt/highcharts/highcharts.js"
end

highcharts_modules accepts URLs and local paths under the same rule: a path is inlined, a URL is referenced.

In a Rails app

If your app already renders charts, it almost certainly ships Highcharts through the asset pipeline. In that case, do nothing: pass the hash from Trevl.render to your existing frontend and let the bundle you already load draw it.

components = Trevl.render(yaml, data: {"rows" => rows})
# hand components.first["highchartsData"] to your Stimulus controller

Only the server-side HTML export needs its own copy. The leanest way to give it one is to reuse the file the asset pipeline already has:

# config/initializers/trevl.rb
Trevl.configure do |c|
  local = Rails.root.join("vendor/javascript/highcharts.js")
  c.highcharts_path = local if local.exist?
end

Without that initializer the export falls back to the CDN, which is fine for anything that renders in a browser with network access.

Development

bin/setup              # install dependencies
bundle exec rspec      # 140 specs
bundle exec standardrb # lint
bin/console            # interactive console

Language Specification

The full TREVL v3.0 specification lives at trevl.trendence.com -- covering component schemas, query definitions, filter operators, computed fields, postprocess patterns, template inheritance, and data source integration.

License

MIT -- see LICENSE.

Highcharts is not covered by that licence and is not distributed with this project. Using it requires a licence from Highsoft; see NOTICE and highcharts.com/license.