code: function (legend, item) {
              var symbolHeight = legend.options.symbolHeight || 12;
  
              item.legendSymbol = this.chart.renderer.rect(
                0,
                legend.baseline - 5 - (symbolHeight / 2),
                legend.symbolWidth,
                symbolHeight,
                legend.options.symbolRadius || 0
              ).attr({
                  zIndex: 3
              }).add(item.legendGroup);
  
          }
docs: Get the series' symbol in the legend
  
  @param {Object} legend The legend object
  @param {Object} item The series (this) or point