code: function _drawLine(v0, v1, color) {
          var p = new Primitive();
  
          p.vertices = [v0, v1];
          p.color = toColor(color);
  
          renderer.addPrimitive(p);
      }
docs: Draws a single line from v0 to v1 with the given color.
  
  Each point is a THREE.Vector3 object.