[ndonkoHenri]

Adds GaugeChart, the chart type fl_chart 1.3.0 introduced, and the fl_chart options the other charts were missing. fl_chart goes from ^1.2.0 to ^1.3.0.

GaugeChart

Progress gauges, activity rings and speedometers, built from:

Part	Classes
Rings, innermost first	GaugeProgressRing (filled up to value, optional bgcolor track), GaugeZonesRing of GaugeZone(start, end, color)
Ticks	GaugeLineTicks, GaugeCircleTicks, with optional labels
Pointers	GaugeNeedlePointer, GaugeCirclePointer
Markers	GaugeMarker, a line with an optional label at one value

The chart takes the scale (min_value, max_value), the arc (start_degree_offset, sweep_angle, direction), ring sizing (default_ring_width, rings_space), animation, interactive, and on_event, which sends a GaugeChartEvent with the touched ring, zone, value and angle.

fch.GaugeChart(
    max_value=100,
    rings=[
        fch.GaugeProgressRing(value=65, width=16, stroke_cap=ft.StrokeCap.ROUND),
        fch.GaugeZonesRing(width=6, zones=[
            fch.GaugeZone(0, 60, ft.Colors.GREEN),
            fch.GaugeZone(60, 85, ft.Colors.AMBER),
            fch.GaugeZone(85, 100, ft.Colors.RED),
        ]),
    ],
    ticks=fch.GaugeLineTicks(count=11),
    pointers=[fch.GaugeNeedlePointer(value=65, length=80)],
)

Design choices worth a look:

• The parts are @ft.value types, not controls. The polymorphic bases carry a _type tag, like ChartPointShape, and the client parses them as maps.
• Validation. Value types aren't validated on their own, so GaugeChart.before_update validates each part. Python raises a ValueError for inputs fl_chart would reject, such as a ring value outside the scale, sweep_angle over 360, a zone ending before it starts, or labels not matching the tick count. Every rule documents a Raises: entry on each property it compares.
• Keyword-only. Every part except GaugeZone is keyword-only, so GaugeNeedlePointer(100) can't set length when value was meant.
• Tick labels are a list of strings, one per tick, since Python can't send fl_chart a label builder.
• Theme-aware defaults. Paint colors default to the theme's on_surface (a progress ring's to primary) instead of fl_chart's black, which disappears in dark mode. A marker label without a style gets 12-pixel text in the marker's color, since fl_chart skips labels that have no style.

Other charts

• PieChart: PieChartSection.corner_radius and stacked PieChartSection.segments (PieChartSectionSegment).
• BarChart: BarChartRod.label (a string or BarChartRodLabel), and label, label_style and gradient on BarChartRodStackItem. A stack item now needs a color or a gradient, so a missing one raises a ValueError instead of breaking the chart.
• LineChart: LineChartData.gradient_area.
• Axes: ChartAxis.label_alignment (outside, on the border, or inside the chart).
• Events: ChartEventType values for secondary-button (right-click) taps and long presses.
• Docs: RadarDataSet.entries gets the Raises: entry its rule was missing.

On the Dart side, each new enum has a named parser like parseStrokeCap, and resolveFlTouchEventType is a switch expression.

Examples, docs and tests

• Examples: gauge_chart/speedometer (a slider moves the needle and the progress ring) and gauge_chart/activity_rings (tapping a ring highlights it), both in the gallery.
• Docs: a GaugeChart page with both examples, type pages for the new classes, and entries in the sidebar, the charts index and the flet-charts README.
• integration_tests/examples/test_gauge_chart.py: the docs image, a slider drag on the speedometer, and a tap on the outer activity ring. Its three goldens are also the docs images.

Testing

• dart analyze and ruff are clean, and the flet-charts unit tests pass.
• The flet-charts integration tests pass on macOS, apart from Matplotlib and Plotly, which I didn't run. Those tests don't run in CI.
• The crocodocs build resolves every link.

Not in this PR

• The BarChart crash on stack items without from_y and the LineChart animation fix (flet-charts#8) are in Fix BarChart stack items without from_y and LineChart point animation #7064, which targets 1.1.x.

Summary by Sourcery

Add GaugeChart support and broaden existing chart capabilities through the fl_chart 1.3.0 upgrade.

New Features:

• Add GaugeChart with configurable rings, ticks, markers, pointers, animation, interaction, and gauge events.
• Add gallery examples and integration coverage for speedometer and activity-ring gauges.

Bug Fixes:

• Reject bar chart stack items that lack both a color and a gradient.

Enhancements:

• Expand pie, bar, line, axis, and chart event APIs with additional fl_chart capabilities, including pie segments, rod labels and gradients, line gradient areas, axis label placement, and secondary-button interactions.
• Upgrade the charts Flutter dependency to fl_chart 1.3.0 and expose the new chart types and supporting value classes.

Documentation:

• Document GaugeChart and its supporting types, and add it to the charts documentation and navigation.

Tests:

• Add integration tests and golden screenshots covering GaugeChart rendering, slider updates, and ring interaction.

Chores:

• Add GaugeChart to the public flet-charts exports and release changelogs.

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