fix(flet-charts): ScatterChart failing to render spots with error ranges

[ndonkoHenri]

Problem

ScatterChartSpot.x_error and y_error were typed Optional[Any] and passed unparsed to fl_chart's ScatterSpot, whose xError/yError are FlErrorRange?. Control.get<T> infers T from the parameter and returns the wire value as is, so any value other than None failed the cast in build():

type '_Map<dynamic, dynamic>' is not a subtype of type 'FlErrorRange?'

Flutter then replaced the whole chart with its error widget, a grey box in release builds, and the message only reached Python as page.on_error. No value could work, so both fields could only ever be None. It's the same defect class as #6684 and #6744.

Changes

• New ChartErrorRange value type with the lower_by and upper_by bounds of FlErrorRange (fl_chart 1.2.0), plus symmetric() and copy(). Negative bounds raise ValueError, since FlErrorRange asserts they are non-negative. x_error/y_error are now typed with it; every non-None value crashed before, so no working code changes.
• Dart: parseFlErrorRange() parses both fields.
• Error bars are drawn in the color of their spot, which replaces the commented-out errorIndicatorData line. fl_chart already draws bars for spots with error ranges (FlErrorIndicatorData.show defaults to true), but its default painter is white, which is invisible on light backgrounds. Spot color is also the default in matplotlib and Plotly.
• The field docstrings note that automatic axis bounds cover spot positions only (fl_chart's ScatterChartHelper ignores error ranges), so bars need explicit min_*/max_* to stay inside the chart.
• New example scatter_chart_with_error_bars (pendulum timings: a symmetric range on x, an asymmetric fastest-to-slowest range on y), shown on the ScatterChart docs page.
• ChartErrorRange docs page and sidebar entry, and changelog entries in CHANGELOG.md and the flet-charts changelog.
• sdk/python/Taskfile.yml: Yarn telemetry is disabled in the docs tasks.

Not in this PR: an API to style the bars (color, width, cap length) through fl_chart's errorIndicatorData, and error ranges for LineChart/BarChart, which fl_chart also supports.

Testing

• A widget test that builds the chart from a MessagePack-shaped Control tree reproduced the cast error before the fix and passed after it. It also checked the bar color and the parser's fallbacks.
• Rendering the chart to PNG in light and dark themes: with fl_chart's default painter the bars were invisible on the light background; spot-colored bars show in both.
• Python: a spot sends {"lower_by": 0, "upper_by": 1.5} (both bounds are always sent, zero included), and negative bounds raise.
• The new example in the macOS test client renders as expected, and the existing scatter_chart integration goldens (image_for_docs, example_1) still match.
• dart analyze, ruff, the Python unit tests (914 passed), and crocodocs generate.

Summary by Sourcery

Fix scatter-chart error-range support and add a public API and documentation for visible error bars.

New Features:

• Add the ChartErrorRange value type for configuring symmetric or asymmetric scatter-plot uncertainty ranges.
• Provide a scatter-chart example demonstrating horizontal and vertical error bars.

Bug Fixes:

• Fix ScatterChart rendering failures when spots include x- or y-axis error ranges.
• Render scatter-chart error bars using each spot’s color so they remain visible across themes.

Enhancements:

• Document error-range axis-bound behavior and expose the new type through the charts package API.

Build:

• Disable Yarn telemetry during documentation tasks.

Documentation:

• Document ChartErrorRange and add the error-bar example to the ScatterChart documentation.

Tests:

• Add coverage for error-range parsing, validation, serialization, rendering, and backward compatibility of existing scatter-chart examples.

[sourcery-ai]

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[Copilot]

Copilot review overview

🟡 Changes recommended

Range validation remains bypassable, and the regression tests claimed in the PR are absent.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Enforce non-negative bounds on assignment and serialization · New
• Add missing ChartErrorRange and parser regression tests · New

What changed in this PR

Fixes ScatterChart error-range rendering and adds a typed Python API with supporting documentation and examples.

Changes:

• Adds ChartErrorRange and parses it into Flutter FlErrorRange.
• Renders spot-colored error bars and adds an example.
• Updates documentation, changelogs, and docs task telemetry settings.

File	Description
CHANGELOG.md	Records the user-facing fix.
sdk/​python/​Taskfile.yml	Disables Yarn telemetry for docs tasks.
.../​scatter_chart_with_error_bars/​main.py	Demonstrates error bars.
.../​scatter_chart_with_error_bars/​pyproject.toml	Defines example metadata.
sdk/​python/​packages/​flet-charts/​CHANGELOG.md	Records the package fix.
flet_charts/​__init__.py	Exports ChartErrorRange.
flet_charts/​scatter_chart_spot.py	Types and documents error ranges.
flet_charts/​types.py	Implements ChartErrorRange.
flutter/​.../​scatter_chart.dart	Parses and renders error ranges.
flutter/​.../​utils/​charts.dart	Adds the range parser.
flutter/​.../​utils/​scatter_chart.dart	Adds the error-bar painter.
website/​docs/​.../​scatterchart.md	Adds the example to documentation.
website/​docs/​.../​charterrorrange.md	Adds API reference documentation.
website/​sidebars.yml	Adds the new type to navigation.

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