When to use

Invoke when the user asks to:

• Create a custom Evidence.dev component ("make a custom chart", "build a reusable Svelte component")
• Create or publish an Evidence.dev plugin
• Extend Evidence.dev with custom functionality beyond built-in components

Evidence Custom Components & Plugins

Level 1 — What are custom components, and when do you need them?

Evidence ships a built-in component library (@evidence-dev/core-components) that covers charts, tables, values, and common UI patterns. Components use HTML-style syntax in Markdown:

<BarChart data={orders_by_month} x="order_month" y="sales_usd" />

Use built-in components for standard charts (Bar, Line, Area, Scatter, etc.), tables, <Value> inline text, reference lines, and annotations. These work out of the box with no extra setup.

Build a custom component when you need to:

• Compose multiple built-in components into a reusable block (e.g. a "KPI card" that combines a Value + sparkline)
• Add UI that Evidence does not provide (custom HTML layouts, bespoke visualisations, interactive inputs)
• Encapsulate a query + its display together (self-contained component)
• Share components across multiple Evidence projects (publish as a plugin)

Custom components are Svelte files placed in the components/ folder at the root of your Evidence project. Evidence discovers them automatically — no registration needed for local components.

Level 2 — Creating a custom component

File structure

.
├── pages/
│   └── index.md
└── components/
    └── Hello.svelte

Passing data via props (the basic pattern)

Define the query in the Markdown page, then pass the result to the component as a prop.

<!-- index.md -->

select 'Canada' as country, 100 as sales_usd union all select 'USA' as country, 200 as sales_usd union all select 'UK' as country, 300 as sales_usd

<Hello myData={sales_by_country} />

<!-- components/Hello.svelte -->
<script>
  export let myData;
  import { BarChart } from '@evidence-dev/core-components';
</script>

<p>Sales by country:</p>
<BarChart data={myData} />

Key rules:

• Use export let propName for every prop the component accepts
• Import any Evidence built-in components explicitly inside the <script> block
• The components/ folder only supports .svelte files — no Markdown inside components

Utility functions available inside custom components

All utilities are imported explicitly. Import only what you need.

Error handling

import checkInputs from '@evidence-dev/component-utilities/checkInputs';
// checkInputs(data, reqCols, optCols) — throws if data empty or columns missing

import { ErrorChart } from '@evidence-dev/core-components';
// <ErrorChart error={error} chartType="My Chart" />

Data manipulation

import getDistinctValues  from '@evidence-dev/component-utilities/getDistinctValues';   // getDistinctValues(data, column)
import getDistinctCount   from '@evidence-dev/component-utilities/getDistinctCount';    // getDistinctCount(data, column)
import getSortedData      from '@evidence-dev/component-utilities/getSortedData';       // getSortedData(data, col, isAsc)
import getColumnSummary   from '@evidence-dev/component-utilities/getColumnSummary';    // getColumnSummary(data, "object"|"array")
import getCompletedData   from '@evidence-dev/component-utilities/getCompletedData';    // fills gaps in a continuous series

Formatting

import { formatValue } from '@evidence-dev/component-utilities/formatting';  // formatValue(value, columnFormat, columnUnitSummary)
import { fmt }         from '@evidence-dev/component-utilities/formatting';  // fmt(value, formatString) — simpler
import formatTitle     from '@evidence-dev/component-utilities/formatTitle'; // formatTitle(column, columnFormat)

Level 3 — Component queries, plugin development, and publishing

Component queries — self-contained data fetching

Instead of passing data from a parent page, components can run their own SQL queries. This is useful for widgets that always fetch the same data (e.g. a schema browser, a row-count badge).

#### Static queries (SQL fixed at component creation time)

<!-- components/TableList.svelte -->
<script>
  import { buildQuery } from '@evidence-dev/component-utilities/buildQuery';
  import { QueryLoad }  from '@evidence-dev/core-components';

  const query = buildQuery('SELECT * FROM information_schema.tables');
</script>

<QueryLoad data={query} let:loaded={tables}>
  <svelte:fragment slot="skeleton" />

  <ul>
    {#each tables as table}
      <li>{table.table_name}</li>
    {/each}
  </ul>
</QueryLoad>

QueryLoad handles execution, loading state, and errors. The let:loaded directive exposes the result under the variable name you choose (prefer a descriptive name over the default loaded).

#### Dynamic queries (SQL changes based on user input / reactive state)

<!-- components/DynamicTableList.svelte -->
<script>
  import { QueryLoad }        from '@evidence-dev/core-components';
  import { getQueryFunction } from '@evidence-dev/component-utilities/buildQuery';
  import { Query }            from '@evidence-dev/sdk/usql';

  let query;
  const queryFunction = Query.createReactive({
    execFn: getQueryFunction(),
    callback: v => query = v
  });

  let limit = 10;
  let schemaName = 'public';

  // Re-runs whenever limit or schemaName change
  $: queryFunction(`
    SELECT * FROM information_schema.tables
    WHERE table_schema = '${schemaName}'
    LIMIT ${limit}
  `);
</script>

<label>Rows: <input type="number" bind:value={limit} min={0} /></label>
<label>Schema: <input type="text" bind:value={schemaName} /></label>

<QueryLoad data={query} let:loaded={tables}>
  <svelte:fragment slot="skeleton" />
  <ul>
    {#each tables as table}
      <li>{table.table_name}</li>
    {/each}
  </ul>
</QueryLoad>

The $: reactive block is Svelte's way of declaring a statement that re-runs when its dependencies change. Query.createReactive wires the result back into the component via the callback.

#### Error slot in QueryLoad

<QueryLoad data={query} let:loaded={tables}>
  <svelte:fragment slot="skeleton" />

  <svelte:fragment slot="error" let:error>
    <div class="text-red-600">
      <h3>Unable to load data</h3>
      <p>{error.message}</p>
    </div>
  </svelte:fragment>

  <!-- main content only renders when query succeeds -->
  <ul>
    {#each tables as table}
      <li>{table.table_name}</li>
    {/each}
  </ul>
</QueryLoad>

Plugin development — sharing components across projects

A plugin is an npm package containing Svelte components. Use the Evidence Labs repo as a starting template.

#### Steps to create and publish a plugin

1. Clone the Labs example repo
2. Replace components in src/lib/ with your own
3. Add demo pages under pages/
4. Set up component exporting (see below)
5. Test with npm run dev
6. Update package.json: set name to your package name, version to 0.0.1
7. Publish: npm publish (requires an npm account)
8. To release updates, bump the version and re-run npm publish

#### Component exporting — Module Exports (recommended)

Add the evidenceInclude flag to every component you want exposed:

<!-- src/lib/ComponentOne.svelte -->
<script context="module">
  export const evidenceInclude = true;
</script>

Create src/lib/index.js and re-export each component:

// src/lib/index.js
export { default as ComponentOne } from './ComponentOne';
export { default as ComponentTwo } from './ComponentTwo';

#### Component exporting — Manifest (alternative, useful for large libraries)

# src/lib/evidence.manifest.yaml
components:
  - ComponentOne
  - ComponentTwo

Plus the same index.js exports above. The manifest takes priority over module exports if both are present.

Installing and registering plugins in an Evidence project

Install

npm install @acme/charting

Register in evidence.config.yaml:

plugins:
  components:
    @evidence-dev/core-components: {}
    @acme/charting: {}

#### Aliases — rename a component on import

components:
  @acme/charting:
    aliases:
      LongNameForAChart: AcmeChart   # use <AcmeChart /> in Markdown

#### Overrides — replace a built-in component

components:
  @evidence-dev/core-components: {}
  @acme/charting:
    overrides:
      - LineChart   # replaces the built-in LineChart with the plugin's LineChart

To override with a differently named component, alias it first then override:

  @acme/charting:
    aliases:
      CustomLineChart: LineChart
    overrides:
      - LineChart

#### Using a generic Svelte library (not an Evidence plugin)

components:
  @evidence-dev/core-components: {}
  carbon-components-svelte:
    provides:
      - Button
      - CodeSnippet

Components must be named exports (import { ComponentName } from 'package'), not default exports.

Data source plugins

Source plugins add new database/connector types to Evidence.

Install and register:

npm install @cool-new-db/evidence-source-plugin

# evidence.config.yaml
plugins:
  components:
    @evidence-dev/core-components: {}
  datasources:
    @cool-new-db/evidence-source-plugin

After registering, restart the dev server and configure the connection at localhost:3000/settings.

Quick reference checklist

In the Markdown page

• [ ] Write the SQL query block with a result name
• [ ] Pass query results to the component as props: <MyComponent data={query_name} />

In the Svelte component

• [ ] Place the file in components/ at the project root
• [ ] Declare props with export let propName
• [ ] Import Evidence built-ins explicitly: import { BarChart } from '@evidence-dev/core-components'
• [ ] For self-contained queries, use buildQuery (static) or Query.createReactive + getQueryFunction (dynamic)
• [ ] Wrap query results in <QueryLoad> to handle loading and error states

When publishing as a plugin

• [ ] Flag each component with export const evidenceInclude = true in a context="module" script block
• [ ] Export all components from src/lib/index.js
• [ ] Set package name and version in package.json before npm publish

Parallelisation

Generally safe — writes to components/*.svelte files. Avoid running alongside other agents editing the same component file.