tool.minDistance = 10;
function onMouseDrag(event) {
var path = new Path();
path.strokeColor = 'black';
var vector = event.delta;
// rotate the vector by 90 degrees:
vector.angle += 90;
// change its length to 5 pt:
vector.length = 5;
path.add(event.middlePoint + vector);
path.add(event.middlePoint - vector);
}