Hello @dale

I make some tries with some points with on same Y positon and the result is ok, the problem

is when the point is no the same Y positon like this:

On the right side itâ€™s good, on the left side there should have been an arc like red line,

and the all original points is on 0 Z position, this arc have some diferent Z point.

this is my code:

```
Rhino.Geometry.Point3d start = new Rhino.Geometry.Point3d(origemX, origemY,
origemY);
Rhino.Geometry.Point3d end = new Rhino.Geometry.Point3d(X, Y, Z );
Rhino.Geometry.Point3d Center = new Rhino.Geometry.Point3d(I, J, K);
var xaxis = start - Center;
var radius = xaxis.Length;
xaxis.Unitize();
var yaxis = Rhino.Geometry.Vector3d.CrossProduct(Rhino.Geometry.Vector3d.ZAxis,
xaxis);
yaxis.Unitize();
Rhino.Geometry.Plane plane = new Rhino.Geometry.Plane(Center, xaxis, yaxis);
var dir = end - Center;
dir.Unitize();
var dot = Rhino.RhinoMath.Clamp(xaxis * dir, -1.0, 1.0);
var angle = System.Math.Acos(dot);
Rhino.Geometry.Plane yplane = new Rhino.Geometry.Plane(plane.Origin, plane.YAxis);
var dist = yplane.ValueAt(end);
if (dist < 0.0)
angle = (System.Math.PI * 2.0) - angle;
Rhino.Geometry.Arc arc = new Rhino.Geometry.Arc(plane, radius, angle);
doc.Objects.AddPoint(start);
doc.Objects.AddPoint(Center);
doc.Objects.AddPoint(end);
```