code: function diff( x, y, scale ) {
  	var f;
  	var s;
  	if ( x !== x || y !== y ) {
  		return NaN;
  	}
  	if (
  		x === PINF ||
  		x === NINF ||
  		y === PINF ||
  		y === NINF
  	) {
  		if ( x === y ) {
  			return NaN;
  		}
  		return PINF;
  	}
  	// If the absolute difference is `0`, then so is the relative difference:
  	if ( x === y ) {
  		return 0.0;
  	}
  	if ( isFunction( scale ) ) {
  		f = scale;
  	} else {
  		if ( scale === void 0 ) {
  			scale = 'max-abs';
  		}
  		f = SCALE[ scale ];
  		if ( f === void 0 ) {
  			throw new Error( 'invalid input argument. Unrecognized/unsupported scale function. Value: `' + scale + '`.' );
  		}
  	}
  	s = f( x, y );
  	if ( s === 0 ) {
  		// Bail. No possible return value which works for all possible scale functions...
  		return NaN;
  	}
  	return abs( (x - y) / s );
  }
docs: RELATIVE DIFFERENCE // 
  FUNCTION: diff( x, y[, scale] )
  Computes the relative difference.
  
  @param {Number} x - first number
  @param {Number} y - second number
  @param {String|Function} [scale='max-abs'] - scale function
  @returns {Number} relative difference