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field = new Field($('container'), $('knob'));

logger = $('logger');

field.addEvent('change', function(position){
    logger.set('text', 'x: ' + position.x + ', y: ' + position.y)
});

<div id="container">
    <div id="knob"></div>
</div>
<p>Position is: <span id="logger"></span></p>