Add DonutChart diagram type

[johnjehiel]

This PR introduces a new DonutChart diagram type, built entirely from Object, Group, and a new DonutArc extended object, which is based on mxgraph.basic.partConcEllipse infographics shape.
The design closely follows the conventions and workflow established by PieChart and BarChart, while extending them with annotation-based labeling for greater flexibility.

Annotation-based design

class AnnotationPosition(Enum):
    INSIDE = 0.6
    MIDDLE = 1.0
    OUTSIDE = 1.4

@dataclass(frozen=True)
class ArcAnnotation:
    name: str = "Annotation"
    formatter: Callable[[str, float, float], str] = (
        lambda label, value, total: f"{value/total:.1%}"
    )
    position: Union[AnnotationPosition, float] = AnnotationPosition.MIDDLE
    text_format: Optional[TextFormat] = None

Instead of a single hard-coded label/value pair, DonutChart uses annotations:

• Each arc can render multiple annotations
• Each annotation controls:
  • Its own formatter
  • Its own radial position
• Supported default positions:
  • INSIDE
  • MIDDLE
  • OUTSIDE
• Custom radial offsets are also supported

This design avoids label/value overlap issues and gives users full control over presentation without workarounds.

Few example usages

Labels inside, values in the middle

chart = DonutChart(
    data=data,
    title="PERSONAL ACCOUNTING",
    ...
    annotations=[
        ArcAnnotation(
            name="Label",
            position=AnnotationPosition.INSIDE,
            formatter=lambda label, value, total: label,
        ),
        ArcAnnotation(
            name="Value",
            position=AnnotationPosition.MIDDLE,
            formatter=lambda label, value, total: f"{value/total:.1%}",
        ),
    ],
)

Labels and values in the middle

chart = DonutChart(
    data=data,
    title="PERSONAL ACCOUNTING",
    ...
    annotations=[
        ArcAnnotation(
            name="Label",
            position=AnnotationPosition.MIDDLE,
            formatter=lambda label, value, total: f"{label} ({value/total:.1%})",
        ),
    ],
)

Labels outside, values middle

chart = DonutChart(
    data=data,
    title="PERSONAL ACCOUNTING",
    ...
    annotations=[
        ArcAnnotation(
            name="Label",
            position=AnnotationPosition.OUTSIDE,
            formatter=lambda label, value, total: label,
        ),
        ArcAnnotation(
            name="Value",
            position=AnnotationPosition.MIDDLE,
            formatter=lambda label, value, total: f"{value/total:.1%}",
        ),
    ],
)

PR Roadmap

• Addition of relevant infographic shapes ✔️
• DiagramType ✔️
• Development Scripts
• Tests
• Documentation / README update

Before proceeding with development scripts, tests and documentation, I’d appreciate a review of the current implementation, especially the annotation based design (which can also be extended to existing pie chart implementation).

[ibirothe]

This is gold! I love it.

Definitely going to add the annotation-based approach to PieChart once this is merged!

[ibirothe]

If there is one thing I could add to this, it would be the option to place the title either above (as in your examples) or in the middle of the donut (as this is something I see a lot, especially when having multiple DonutCharts alongside).