Feature/7078 make changes to allow for decimal values for sequence numbers

[milesspencer35]

📑 Summary

Added support for sequence numbers in sequence diagrams to use decimal values up to the hundredths place, and enabled incrementing using decimal values to the same precision. Other PR had issues, so this is a new one.

Resolves #7078

📏 Design Decisions

Describe the way your implementation works or what design decisions you made if applicable.

• Updated the Jison parser for sequence diagrams to accept decimal values to the hundredths place.
• Updated how the sequenceIndex and sequenceIndexStep are added to remove floating-point errors.
• Decrease the font size once a sequence number gets sufficiently long and spills out of the circle. I decided this was the best course of action because it was a simple fix for a rare use case.

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Latest commit: 50b2166

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❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 3.55%. Comparing base (717d3b3) to head (50b2166).
⚠️ Report is 136 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../mermaid/src/diagrams/sequence/sequenceRenderer.ts	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #7174      +/-   ##
==========================================
- Coverage     3.55%   3.55%   -0.01%     
==========================================
  Files          474     473       -1     
  Lines        47453   47504      +51     
  Branches       730     731       +1     
==========================================
  Hits          1687    1687              
- Misses       45766   45817      +51     

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Files with missing lines	Coverage Δ
.../mermaid/src/diagrams/sequence/sequenceRenderer.ts	0.06% <0.00%> (-0.01%)	⬇️

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Build	Status	Details	Updated (UTC)
default (Inspect)	⚠️ Changes detected (Review)	1 added	Nov 18, 2025, 5:45 PM

[Copilot]

Pull request overview

This PR adds support for decimal sequence numbers in sequence diagrams, allowing both the start value and increment value for the autonumber directive to use decimal numbers with precision up to the hundredths place (e.g., autonumber 10.1 0.01). This enhancement provides more flexibility for users who need fine-grained control over sequence numbering.

Key Changes

• Updated the Jison parser to accept decimal values (tenths or hundredths place) for sequence numbers
• Implemented floating-point precision handling using rounding to prevent arithmetic errors
• Added dynamic font sizing for sequence number display to handle longer numeric strings

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison	Updated NUM token regex to match decimal numbers with up to 2 decimal places
packages/mermaid/src/diagrams/sequence/sequenceRenderer.ts	Added Math.round() logic to handle floating-point precision and dynamic font sizing for sequence numbers
packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js	Added unit tests for decimal value parsing, validation, and increment calculations
cypress/integration/rendering/sequencediagram.spec.js	Added E2E test for visual rendering of decimal sequence numbers
packages/mermaid/src/docs/syntax/sequenceDiagram.md	Added documentation for the new decimal value support in autonumber
docs/syntax/sequenceDiagram.md	Added documentation for the new decimal value support in autonumber
.changeset/dark-bats-build.md	Added changeset documenting this minor feature addition

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[omkarht]

sequenceDiagram
    autonumber 10
    Server->>Client: Snapshot for tick 10
    autonumber 10.3
    Client->>Server: Command for tick 10.3
    Server->>Client: Test Command 
    Client->>Server: New Test Command 

Great work @milesspencer35 on enabling decimal autonumber seeds — the formatting and parsing improvements look solid.

One thing I wanted to confirm: currently, autonumber increases by whole numbers even when seeded with a decimal (e.g., 10.3 → 11.3 → 12.3). There’s no fractional stepping, and decimals only affect the display formatting.

Was this an intentional, or should the increment behavior differ depending on the decimal precision? (Example: some users may expect 10.3 → 10.4 → 10.5 rather than 11.3.)

Just checking whether the current behavior aligns with the feature expectations. Thanks !

[milesspencer35]

@omkarht thank you for taking the time to look at my PR.

Currently, how autonumber works is that you can set a start and increment value. For example, autonumber 10 5 makes it so that the sequence number starts at 10 and increases by 5. If no increment value is set, then the default increment value is 1.

This PR allows for both the start and increment values to be decimals, with the default increment number still being 1. So, for the example you gave above, if you wanted your diagram to increase 10.3 -> 10.4 -> 10.5, you would do autonumber 10.3 0.1.

So I believe the use case you are thinking of is covered! Please let me know if I can clarify more or if this doesn't answer your question!

[omkarht]

Thanks for the clarification @milesspencer35, that makes the behaviour clear.

It’s great that both the start and increment values can be decimals (e.g., autonumber 10.3 0.1), allowing fractional stepping when configured. That aligns with the expected use case.

Looks good from my side 👍