Charts-143: Add Sparkline chart component

[annacmc]

Proposed changes:

• Add new Sparkline chart component for inline trend visualization
• Add display property to AxisOptions type for controlling axis visibility
• Use existing gridVisibility prop for grid control
• Sparkline provides a simplified API (number[] data) while using LineChart internally
• Handle edge cases: empty data, single point (circle), negative values, flat lines
• Support theme integration and responsive behavior via withResponsive HOC

Implementation Details:

Types changes:

• New display?: boolean property in AxisOptions type (default: true) - controls individual axis rendering

LineChart changes:

• Uses gridVisibility prop for grid control (already existed in BaseChartProps)
• Uses options.axis.x.display and options.axis.y.display for axis control

Sparkline wrapper:

• Transforms number[] to SeriesData[] format for LineChart
• Preconfigures: gridVisibility="none", options.axis.x/y.display=false, withTooltips={false}, showLegend={false}
• Uses monotone curve interpolation
• Handles edge cases (empty data, single point) with custom rendering

Other information:

• [x] Have you written new tests for your changes, if applicable?
• [x] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Does this pull request change what data or activity we track or use?

No, it does not.

Testing instructions:

• Go to Storybook -> JS Packages -> Charts -> Types -> Line Chart -> Sparkline
• Test available stories:
  • Default: Basic sparkline with controls
  • Responsive: Container-based responsive sizing
  • Dashboard: Metric card integration example
  • EmptyData: Empty state handling
  • SinglePoint: Single point rendering (circle)
  • TwoPoints: Minimal line rendering
• Test LineChart with minimal rendering:
  • Use gridVisibility="none" to hide grid
  • Use options={{ axis: { x: { display: false }, y: { display: false } } }} to hide axes

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/js-packages/charts/src/charts/line-chart/line-chart.tsx	136/141 (96.45%)	1.42%	-2 💚

3 files are newly checked for coverage.

File	Coverage
projects/js-packages/charts/src/charts/sparkline/sparkline.tsx	34/34 (100.00%) 💚
projects/js-packages/charts/src/charts/sparkline/index.ts	0/0 (—%) 🤷

Full summary · PHP report · JS report

[adamwoodnz]

Thanks for the PR 🙂

I'm on support rotation this week so I'd like to review properly next week if possible.

One initial thought is that this looks a lot like a widget based on a LineChart (with no axes displayed), rather than a new primitive chart type. Did you consider that?

[annacmc]

Thanks that would be amazing if you have some space to look next week (either at this or I do have ~3 components I am attempting to add in this week) - I have considered a line chart widget, but put that idea down at the time when I pursued this direction, not for any great reason. You might be onto something here though I will definitely look into that possibility once more.

[annacmc]

OK I've had a bit of a play around - I've refactored Sparkline to use LineChart internally with new showAxes and showGrid props, so it's now a preconfigured variant rather than a separate primitive - what do you think of this approach @adamwoodnz ?