var path             = "M150 0 L75 200 L225 200 Z";    // an SVG path
    pathAnimator,
    startFromPercent = 10,  // start from 10% of the path
    stopAtPercent    = 70;  // stop at 70% of the path (which will then call the onDone function callback)


// initiate a new pathAnimator object
pathAnimator = new PathAnimator( path, {
    duration : 4, // seconds that will take going through the whole path
    step     : step,
    easing   : function(t){ return t*(2-t) },
    onDone   : finish(this)
});

pathAnimator.start( startFromPercent, stopAtPercent  );

function step( point, angle ){
    // do something every "frame" with: point.x, point.y & angle
}

function finish(){
    // do something when animation is done
}