попробуйте с этим
var chart = new Highcharts.Chart({
chart: {
renderTo: 'container'
},
series: [{
data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0, 135.6, 148.5, 216.4, 194.1, 95.6, 54.4]
}]
}, function(chart){
chart.series[0].legendLine.attr({
stroke: 'red'
});
chart.series[0].legendSymbol.attr({
stroke: 'red',
fill: 'red'
});
});