feat: Add FlDotImagePainter for custom image dot markers

[InsoooooooooJANG]

Description

Adds FlDotImagePainter, an implementation of FlDotPainter that draws a dart:ui Image as the dot marker, instead of the built-in circle, square or cross shapes.

The image is centered on the spot and scaled to fit inside a size x size square, keeping its aspect ratio. Since FlDotPainter.draw() is synchronous, the image has to be decoded before the painter is created — loading is left to the user (asset, network, generated), and LineChartSample14 shows one way to do it.

final data = await rootBundle.load('assets/dot.png');
final buffer = await ui.ImmutableBuffer.fromUint8List(data.buffer.asUint8List());
final codec = await PaintingBinding.instance.instantiateImageCodecWithSize(buffer);
final frame = await codec.getNextFrame();

final painter = FlDotImagePainter(image: frame.image, size: 20);

LineChartBarData(
  spots: spots,
  dotData: FlDotData(
    getDotPainter: (spot, percent, barData, index) => painter,
  ),
)

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///.
• I have updated/added relevant examples in example.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #2050

[imaNNeo]

Please use canvasWrapper.drawImage() instead of paintImage. This way, we keep the consistency and testability. Just like here:

fl_chart/lib/src/chart/base/axis_chart/axis_chart_painter.dart

Lines 296 to 305 in c83aa90

	if (line.image != null) {
	final centerX = line.image!.width / 2;
	final centerY = line.image!.height / 2;
	final centeredImageOffset = Offset(centerX, to.dy - centerY);
	canvasWrapper.drawImage(
	line.image!,
	centeredImageOffset,
	_imagePaint,
	);
	}

[InsoooooooooJANG]

paintImage is gone. I couldn't route it through CanvasWrapper though, and I'd rather check the direction with you than guess:

• FlDotPainter.draw only receives a raw Canvas — CanvasWrapper.drawDot unwraps it at canvas_wrapper.dart:123, and that's the only call site.
• CanvasWrapper isn't exported from lib/fl_chart.dart, so changing draw to take one would make FlDotPainter impossible for users to implement without importing package:fl_chart/src/.... That's a public breaking change.
• Canvas.drawImage (the one in the code you linked) has no scaling — it draws at native size. Using it directly would silently ignore FlDotImagePainter.size.
• The raw-Canvas shape also seems to be the convention for user-extensible painters: GaugeTickPainter, GaugeMarkerPainter and GaugePointerPainter all take a Canvas and call canvas.drawLine/save/translate directly, as do the three built-in dot painters and FlSimpleErrorPainter.

So I dropped paintImage for canvas.drawImageRect with applyBoxFit(BoxFit.contain, ...), which keeps both size and the aspect ratio working. For the testability half of your point: draw is now unit-tested against MockCanvas and asserts the exact source/destination rects, the same way FlSimpleErrorPainter.draw is tested in axis_chart_data_test.dart.

If you'd rather have CanvasWrapper in the painter interfaces, I'm glad to do it — I just think it deserves its own breaking PR that exports canvas_wrapper.dart and converts all five painter interfaces at once, rather than changing only this one. Let me know which you prefer.

[imaNNeo]

You also need to write some unit-tests in the end.

[InsoooooooooJANG]

Thanks for the review, and sorry for the long silence on this one. All four points are addressed and main is merged in, so the conflict is gone.

Unit tests — added a FlDotImagePainter group to test/chart/base/axis_chart/axis_chart_data_test.dart covering equality, getSize, mainColor, same-type and cross-type lerp, the inherited hitTest, and draw (asserting the exact src/dst rects that come out of applyBoxFit, plus that a square image fills the whole dot). MockCanvas was already generated for that file, so no make codeGen needed.

One change beyond the review: mainColor was returning Colors.transparent, which would render scatter-chart tooltip text invisible (it feeds the text color at scatter_chart_data.dart:577). It's now a constructor field defaulting to Colors.green, matching FlDotCirclePainter.color.

I also dropped example/assets/icons/ic_meal.png — the sample loads the existing image_annotation.png, so that file was never referenced.

Verified with make runTests (673 passing) and make checkFormat, and I ran the example on web to confirm the sample actually renders.

Heads-up on CI: make analyze is currently failing on main itself — the run for f3a5758 is red with 167 pre-existing EquatableMixin is deprecated infos coming from a newer equatable resolving on current stable. So the red check here isn't from these changes. Want me to send a separate PR that switches those over to Equatable?