ah .. I found a solution ..
---
var linex = view.create('line3d', [
() => JXG.Math.Geometry.meet3Planes(plane.D3.normal, plane.D3.d, planex.D3.normal, planex.D3.d, JXG.Math.crossProduct(plane.D3.normal, planex.D3.normal), 0),
() => JXG.Math.Geometry.meetPlanePlane(plane.D3.dir1, plane.D3.dir2, planex.D3.dir1, planex.D3.dir2)],
{ strokeColor: JXG.palette.red, strokeWidth: 2, strokeOpacity: 0.8 });
var liney = view.create('line3d', [
() => JXG.Math.Geometry.meet3Planes(plane.D3.normal, plane.D3.d, planey.D3.normal, planey.D3.d, JXG.Math.crossProduct(plane.D3.normal, planey.D3.normal), 0),
() => JXG.Math.Geometry.meetPlanePlane(plane.D3.dir1, plane.D3.dir2, planey.D3.dir1, planey.D3.dir2)],
{ strokeColor: JXG.palette.green, strokeWidth: 2, strokeOpacity: 0.8 });
var linez = view.create('line3d', [
() => JXG.Math.Geometry.meet3Planes(plane.D3.normal, plane.D3.d, planez.D3.normal, planez.D3.d, JXG.Math.crossProduct(plane.D3.normal, planez.D3.normal), 0),
() => JXG.Math.Geometry.meetPlanePlane(plane.D3.dir1, plane.D3.dir2, planez.D3.dir1, planez.D3.dir2)],
{ strokeColor: JXG.palette.blue, strokeWidth: 2, strokeOpacity: 0.8 });
---