Start a diagram

Paste or drop Mermaid you already have, open an example, or start a new blank diagram.

Paste Mermaid text

Copy Mermaid text from anywhere, open the diagram page and press ⌘V (Mac) or Ctrl+V (Windows). You can also right-click the diagram and choose Paste.

Line9 replaces the diagram on the page with the one you pasted. To go back, click Undo in the message that appears.

You can paste a whole chat reply or Markdown document: Line9 uses the first Mermaid diagram in it. Pasting a Line9 link opens the diagram it links to.

Drop or open a file

Drag a file onto the diagram page, or open the menu in the top-left corner and choose Diagram ▸ Open from file…. Line9 opens:

  • .mmd and .mermaid files
  • .md files that contain a Mermaid diagram

With Line9 Pro, Diagram ▸ Open… finds any diagram your team has published.

Open an example

Choose Diagram ▸ Examples and pick one. The examples are grouped by subject, from business processes to architecture. Line9 opens a copy for you to change.

The menu open at Diagram, then Examples, which lists Parcel Delivery and seven groups of examples.

You can also browse the examples, with their Mermaid text, in the flowchart examples library.

Start a new blank diagram

Choose Diagram ▸ New diagram, then Flowchart or Sequence diagram. The text opens on the left, ready for you to type.

With Line9 Pro, a new flowchart opens in the Line9 editor instead. The editor tutorial builds one step by step.

Have an AI draw it

Connect Line9 to ChatGPT, Claude or another chat app, and it draws the diagrams you ask for with Line9. See Line9 in chat apps. Developers using a coding agent, such as Claude Code or Codex, can use the Line9 CLI.

Without Line9 connected, ask the AI for a Mermaid flowchart, then paste its reply onto the diagram page.

Keep your diagram

Line9 keeps your latest diagram in this browser and opens it again when you come back. Opening or starting another diagram replaces it.

To keep a diagram, copy a link, export it or publish it. See Share a link, image or text.

Two diagram types: flowcharts and sequence diagrams

Line9 lays out two types of Mermaid diagram.

A flowchart shows the steps of a process and the decisions between them.

The expense approval flowchart, from the employee's claim, through approval, to payment.

A sequence diagram shows who sends what to whom, in order.

A sequence diagram of an expense claim passing between the employee, the expenses app, the line manager and Finance.

Line9 opens other types of Mermaid diagram too, such as pie charts and Gantt charts. Mermaid draws these, and the diagram page shows a Mermaid label at the top.

Diagram size limit

Line9 lays out diagrams of up to 100 nodes. Mermaid draws a larger diagram instead, with the same Mermaid label. A diagram that size is hard to read, so you may want to split it into several smaller ones.

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