Rotate a PolylineVolumeGeometry


I am an absolute beginner with using primitives in Cesium and tried constructing a 3D arc which worked
nicely using PolylineVolumeGeometry. However, my arc is on the surface now and I would like to rotate it 90 degrees. I tried setting the modelMatrix of the geometry but I am doing something wrong with the transformation (as the geometry is rotated around the center of the earth).

I am clearly doing something wrong here and would be immensely grateful for some help to get me off the ground!

Code is here:

Thanks so much.