c3/docs/js/samples/simple_regions.js

var chart = c3.generate({
    data: {
        columns: [
            ['data1', 30, 200, 100, 400, 150, 250],
            ['data2', 50, 20, 10, 40, 15, 25]
        ],
        regions: {
            'data1': [{'start':1, 'end':2, 'style':'dashed'},{'start':3}], // currently 'dashed' style only
            'data2': [{'end':3}]
        }
    }
});