I'm not sure to understand what you are trying to do.
For each point in the tract, you want to show its coordinates as a caption?
It is not supported right now.
A workaround would be to loop around all the points in your tract, and for each of them create a sphere with the appropriate caption.
for ( var j = 0; j < numberOfPoints - 1; j++) {
var currentPoint = fibers.points.get(j);
var currentColor = fibers.colors.get(j);
var newSphere = new X.sphere();
newSphere.center = [currentPoint[0], currentPoint[1], currentPoint[2]];
newSphere.color = [currentColor[0], currentColor[1], currentColor[2]];
newSphere.radius = .1;
newSphere.caption = 'x: '+String(currentPoint[0])+'<br>y: '+String(currentPoint[1])+'<br>z: '+String(currentPoint[2]);
r.add(newSphere);
}