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Adding a New Diagram/Chart 📊


Please refer to the following PRs on how to use Langium to add a new diagram grammar.


The below steps are a work in progress and will be updated soon.

Step 1: Grammar & Parsing

Step 2: Rendering

Write a renderer that given the data found during parsing renders the diagram. To look at an example look at sequenceRenderer.js rather than the flowchart renderer as this is a more generic example.

Place the renderer in the diagram folder.

Step 3: Detection of the new diagram type

The second thing to do is to add the capability to detect the new diagram to type to the detectType in diagram-api/detectType.ts. The detection should return a key for the new diagram type. This key will be used to as the aria roledescription, so it should be a word that clearly describes the diagram type. For example, if your new diagram uses a UML deployment diagram, a good key would be "UMLDeploymentDiagram" because assistive technologies such as a screen reader would voice that as "U-M-L Deployment diagram." Another good key would be "deploymentDiagram" because that would be voiced as "Deployment Diagram." A bad key would be "deployment" because that would not sufficiently describe the diagram.

Note that the diagram type key does not have to be the same as the diagram keyword chosen for the grammar, but it is helpful if they are the same.

Common parts of a diagram

There are a few features that are common between the different types of diagrams. We try to standardize the diagrams that work as similar as possible for the end user. The commonalities are:

  • Directives, a way of modifying the diagram configuration from within the diagram code.
  • Accessibility, a way for an author to provide additional information like titles and descriptions to people accessing a text with diagrams using a screen reader.
  • Themes, there is a common way to modify the styling of diagrams in Mermaid.
  • Comments should follow mermaid standards

Here are some pointers on how to handle these different areas.


Mermaid automatically adds the following accessibility information for the diagram SVG HTML element:

  • aria-roledescription
  • accessible title
  • accessible description


The aria-roledescription is automatically set to the diagram type and inserted into the SVG element.

See the definition of aria-roledescription in the Accessible Rich Internet Applications W3 standard.

accessible title and description

The syntax for accessible titles and descriptions is described in the Accessibility documentation section.

The functions for setting title and description are provided by a common module. This is the import in flowDb.js:

import {
  clear as commonClear,
} from '../../commonDb';

The accessibility title and description are inserted into the SVG element in the render function in mermaidAPI.


Mermaid supports themes and has an integrated theming engine. You can read more about how the themes can be used in the docs.

When adding themes to a diagram it comes down to a few important locations in the code.

The entry point for the styling engine is in src/styles.js. The getStyles function will be called by Mermaid when the styles are being applied to the diagram.

This function will in turn call a function your diagram should provide returning the css for the new diagram. The diagram specific, also which is commonly also called getStyles and located in the folder for your diagram under src/diagrams and should be named styles.js. The getStyles function will be called with the theme options as an argument like in the following example:

const getStyles = (options) =>
    .line {
      stroke-width: 1;
      stroke: ${options.lineColor};
      stroke-dasharray: 2;
    // ...

Note that you need to provide your function to the main getStyles by adding it into the themes object in src/styles.js like in the xyzDiagram in the provided example:

const themes = {
  'flowchart-v2': flowchart,

The actual options and values for the colors are defined in src/theme/theme-[xyz].js. If you provide the options your diagram needs in the existing theme files then the theming will work smoothly without hiccups.