partial class Examples
{
  public static Result Leader(RhinoDoc doc)
  {
    var points = new Point3d[]
    {
      new Point3d(1, 1, 0),
      new Point3d(5, 1, 0),
      new Point3d(5, 5, 0),
      new Point3d(9, 5, 0)
    };

    var xy_plane = Plane.WorldXY;

    var points2d = new List<Point2d>();
    foreach (var point3d in points)
    {
      double x, y;
      if (xy_plane.ClosestParameter(point3d, out x, out y))
      {
        var point2d = new Point2d(x, y);
        if (points2d.Count < 1 || point2d.DistanceTo(points2d.Last<Point2d>()) > RhinoMath.SqrtEpsilon)
          points2d.Add(point2d);
      }
    }

    doc.Objects.AddLeader(xy_plane, points2d);
    doc.Views.Redraw();
    return Result.Success;
  }
}

import rhinoscriptsyntax as rs

def RunCommand():
    points = [(1,1,0), (5,1,0), (5,5,0), (9,5,0)]
    rs.AddLeader(points)

if __name__ == "__main__":
    RunCommand()