feat(charts): let LineChart pin the y-axis ceiling

[gitlawr]

Two small LineChart capabilities, both driven by metric charts that plot
ratio and sparse time series.

yAxisMax

A new yAxisMax prop pins the y-axis ceiling. Ratio charts (hit rate,
usage ratio) need a fixed 0–100 axis: with auto-scaling, a series
hovering at 59% fills the frame and reads as saturation. The value is
passed through as yAxis.max ?? undefined, so a null from an untyped
caller falls back to ECharts' auto-scaling.

Per-series showSymbol

Series data can now opt back into symbol rendering: the shared line
config sets showSymbol: false, and previously overrode any
series-provided value. A series with gaps can carry isolated points no
line segment reaches (e.g. a single sample from one traffic burst on a
wide time window) — the caller marks those points with per-item
symbol/symbolSize and sets showSymbol on the series, and they
render instead of existing only in the hover tooltip. Continuous series
are unaffected.

Both are additive; existing callers see no behavior change.

[gemini-code-assist]

Code Review

This pull request introduces a new optional yAxisMax prop to the LineChart component, allowing users to pin the y-axis ceiling. The feedback suggests using the nullish coalescing operator (yAxisMax ?? undefined) when assigning this value to the ECharts configuration to safely handle null values and ensure proper auto-scaling fallback.

[gitlawr]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new yAxisMax prop to the LineChart component and its type definitions, allowing users to pin the y-axis ceiling. It also updates the series mapping to preserve individual item showSymbol configurations. The reviewer suggested a minor cleanup to remove a redundant nullish coalescing operator (?? undefined) when setting the max property on the y-axis, as yAxisMax is already typed as optional.