feat: add matplotlib requirements

[akihikokuroda]

Pull Request

Issue

Fix: #1120

Description

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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Adding a new component, requirement, sampling strategy, or tool?

If your PR adds or modifies one of the types below, check the matching box. A checklist of type-specific review items will be posted as a comment.

• Component
• Requirement
• Sampling Strategy
• Tool

NOTE: Please ensure you have an issue that has been acknowledged by a core contributor and routed you to open a pull request against this repository. Otherwise, please open an issue before continuing with this pull request.

[github-actions]

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Requirement PR Checklist

Use this checklist when adding or modifying requirements in mellea/stdlib/requirements/.

Base Class

• Extends appropriate base class:
  • Requirement - standard requirement
  • ALoraRequirement - uses specialized Intrinsic/Adapter for generation-based validation

Validation Logic

• validation_fn defined (if using Python-based validation)
  • re-usable functionality within the validation_fn should be separated out into mellea/stdlib/tools/
• validate returns a ValidationResult with
  • a thunk and context if using a backend to generate
  • a specific reason and score when possible

Integration

• Requirement exported in mellea/stdlib/requirements/__init__.py or, if you are adding a library of requirements, from your sub-module

[ajbozarth]

Some feedback from Claude — follow-ups inline.

[planetf1]

A few additional findings alongside @ajbozarth's review.

[ajbozarth]

LGTM — all 17 review threads addressed, tests pass locally (24 passed, 1 xfailed).

Re: the xfail on test_valid_dependencies_available — you're right, my earlier comment was wrong. matplotlib isn't in pyproject.toml, leave xfail as-is.

Two small nits inline, neither blocking.