Hello,

To the best of my knowledge setting the radius and the innerradius of the doughnut is not supported functionality. The only way I think this is possible is using scale transformations e.g.:

Dougnut 2D:

<igCA:XamChart.Scene>
                <igCA:Scene>
                    <igCA:Scene.GridArea>
                        <igCA:GridArea>
                            <igCA:GridArea.RenderTransform>
                                   <ScaleTransform  ScaleX="1" ScaleY="1"/>
                            </igCA:GridArea.RenderTransform>
                        </igCA:GridArea>
                    </igCA:Scene.GridArea>
                </igCA:Scene>
</igCA:XamChart.Scene>

or

<igCA:XamChart.Scene>
                <igCA:Scene>
                    <igCA:Scene.GridArea>
                          <igCA:GridArea MarginType="Percent" Margin="5" />
                    </igCA:Scene.GridArea>
                </igCA:Scene>
</igCA:XamChart.Scene>

Doughnut3D:

<igCA:XamChart x:Name="xamChart1" View3D="True">
         <igCA:XamChart.Transform3D>
                <Transform3DGroup>
                    <ScaleTransform3D ScaleX="2" ScaleY="2" ScaleZ="0.3" />
                    <RotateTransform3D>
                        <RotateTransform3D.Rotation>
                            <AxisAngleRotation3D Angle="0" />
                        </RotateTransform3D.Rotation>
                    </RotateTransform3D>
                </Transform3DGroup>
       </igCA:XamChart.Transform3D>
</igCA:XamChart>

If you would like to add properties like radius and innerradius in the next release please submit a feature request here

Regards Vlad