Stop a percentage Y axis expanding past 100

[MindFreeze]

Proposed change

A percentage has a real ceiling the way zero is a real floor, but the gap that keeps series off the plot edges did not know that — so a battery reading 20–100% rounded out to an axis labelled up to 120%, and one sitting flat at 100% reached 160%. This recognises the unit at the two line-chart call sites and holds the axis at 100, mirroring the zero clamp already there. It applies only while the data stays under the ceiling, since power factor is also reported in % and is signed.

Stacked on #53821, which introduced the gap and the zero clamp — review that one first.

Screenshots

A battery sitting at 99%.

Before — the axis runs to 160% and roughly 40% of the plot is values a battery cannot report:

After — the axis stops at 100:

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion: Guarantee a Y-axis gap on history and statistics charts #53821
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to backend pull request:

Why the unit rather than the device class

device_class would be the more precise signal, but it is not available at either call site. LineChartUnit carries it, yet state-history-charts never forwards it, and it is only populated when split_device_classes is on — which the more info dialog and the history graph card both leave off. StatisticsMetaData has no device_class field at all. The unit string is the only signal both callers already hold, and every existing percentage check in the frontend compares the bare "%" literal.

Why the clamp is conditional

% is not a 0–100 unit in general: power_factor is reported in % and core allows it to be signed. The clamp therefore fires only when the widened maximum would actually cross 100, exactly like the zero clamp only fires when the widened minimum would cross zero. A series that genuinely reads 140% is left alone.

A battery that reaches 100% then sits flush against the top edge, as the second screenshot shows. That is the intended symmetry with 0 rather than a regression — an area fill is anchored at the axis minimum, so nothing collapses at the top, and labelling an axis to 160% is the worse of the two.

Checklist

• I understand the code I am submitting and can explain how it works.
• The code change is tested and works locally.
• There is no commented out code in this PR.
• I have followed the perfect PR recommendations
• Any generated code has been carefully reviewed for correctness and compliance with project standards.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

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• I have reviewed two other open pull requests in this repository.