code: function onData() {
  		var flg = false,
  			diff,
  			oldVal;
  
  		/**
  		* FUNCTION: onData( newVal, encoding, clbk )
  		*	Data event handler. Calculates the difference.
  		*
  		* @private
  		* @param {Number} newVal - new streamed data value
  		* @param {String} encoding
  		* @param {Function} clbk - callback to invoke after calculating the difference. Function accepts two arguments: [ error, chunk ].
  		*/
  		return function onData( newVal, encoding, clbk ) {
  			if ( flg ) {
  				diff = newVal - oldVal;
  				oldVal = newVal;
  				clbk( null, diff );
  				return;
  			}
  			// Do not calculate a difference for the first streamed value...
  			oldVal = newVal;
  			flg = true;
  			clbk();
  		}; // end FUNCTION onData()
  	}
docs: FUNCTIONS // 
  FUNCTION: onData()
  Returns a callback which calculates the difference between successive streamed data values.
  
  @private
  @returns {Function} callback