 includes the top left corner
                        .add(new go.PathFigure(radius, radius)) // and the bottom right corner of the whole circular area
                        .add(
                          new go.PathFigure(p0.x - radius, p0.y - radius)
                            .add(
                              new go.PathSegment(
                                go.SegmentType.Arc,
                                a0,
                                sweep,
                                0,
          