code: function defaultFormatter() {
  			var pThis = this,
  				items = pThis.points || splat(pThis),
  				xAxis = items[0].series.xAxis,
  				x = pThis.x,
  				isDateTime = xAxis && xAxis.options.type === 'datetime',
  				useHeader = isString(x) || isDateTime,
  				s;
  
  			// build the header
  			s = useHeader ?
  				['<span style="font-size: 10px">' +
  				(isDateTime ? dateFormat('%A, %b %e, %Y', x) :  x) +
  				'</span>'] : [];
  
  			// build the values
  			each(items, function (item) {
  				s.push(item.point.tooltipFormatter(useHeader));
  			});
  			return s.join('<br/>');
  		}
docs: In case no user defined formatter is given, this will be used