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;
  }
}

Partial Friend Class Examples
  Public Shared Function Leader(ByVal doc As RhinoDoc) As Result
	Dim points = New Point3d() {
		New Point3d(1, 1, 0),
		New Point3d(5, 1, 0),
		New Point3d(5, 5, 0),
		New Point3d(9, 5, 0)
	}

	Dim xy_plane = Plane.WorldXY

	Dim points2d = New List(Of Point2d)()
	For Each point3d In points
	  Dim x As Double = Nothing, y As Double = Nothing
	  If xy_plane.ClosestParameter(point3d, x, y) Then
		Dim point2d = New Point2d(x, y)
		If points2d.Count < 1 OrElse point2d.DistanceTo(points2d.Last()) > RhinoMath.SqrtEpsilon Then
		  points2d.Add(point2d)
		End If
	  End If
	Next point3d

	doc.Objects.AddLeader(xy_plane, points2d)
	doc.Views.Redraw()
	Return Result.Success
  End Function
End Class

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()