ElementStacks

HTML

<script src="http://highcharts.com/js/testing.js"></script>
<div id="container" style="height: 400px"></div>

CSS

body {
    font-family: Arial, Verdana, sans-serif;
}

JavaScript

var chart = new Highcharts.Chart({
    chart: {
        renderTo: 'container'
    },
    xAxis: {
        categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
    },
    
    plotOptions: {
        series: {
            states: {
                hover: {
                    enabled: true,
                    lineWidth: 5
                }
            }
        }
    },
    
    series: [{
        color: 'blue',
        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]        
    }]
});

series = chart.series[0];

series.tracker.on('mouseover', function(){
    series.graph.attr('stroke', 'red');
})
series.tracker.on('mouseout', function(){
    series.graph.attr('stroke', 'blue');
})