code: function () {
              var series = this,
                seriesOptions = series.options,
                chart = series.chart,
                axisOptions;
  
              each(series.axisTypes || [], function (AXIS) { // repeat for xAxis and yAxis
  
                  each(chart[AXIS], function (axis) { // loop through the chart's axis objects
                      axisOptions = axis.options;
  
                      // apply if the series xAxis or yAxis option mathches the number of the
                      // axis, or if undefined, use the first axis
                      if ((seriesOptions[AXIS] === axisOptions.index) ||
                        (seriesOptions[AXIS] !== UNDEFINED && seriesOptions[AXIS] === axisOptions.id) ||
                        (seriesOptions[AXIS] === UNDEFINED && axisOptions.index === 0)) {
  
                          // register this series in the axis.series lookup
                          axis.series.push(series);
  
                          // set this series.xAxis or series.yAxis reference
                          series[AXIS] = axis;
  
                          // mark dirty for redraw
                          axis.isDirty = true;
                      }
                  });
  
                  // The series needs an X and an Y axis
                  if (!series[AXIS] && series.optionalAxis !== AXIS) {
                      error(18, true);
                  }
  
              });
          }
docs: Set the xAxis and yAxis properties of cartesian series, and register the series
  in the axis.series array