code: function (tooltip) {
              var items = this.points || splat(this),
                series = items[0].series,
                s;
  
              // build the header
              s = [tooltip.tooltipHeaderFormatter(items[0])];
  
              // build the values
              each(items, function (item) {
                  series = item.series;
                  s.push((series.tooltipFormatter && series.tooltipFormatter(item)) ||
                    item.point.tooltipFormatter(series.tooltipOptions.pointFormat));
              });
  
              // footer
              s.push(tooltip.options.footerFormat || '');
  
              return s.join('');
          }
docs: In case no user defined formatter is given, this will be used. Note that the context
  here is an object holding point, series, x, y etc.