var chart = c3.generate({
    data: {
        columns: [
            ['sample', 30, 200, 100, 400, 150, 250]
        ]
    },
    axis: {
        x: {
            type: 'category',
            categories: ['a', 'b', 'c', 'd', 'e', 'f'],
        }
    },
    grid: {
        x: {
            //lines: [{value: 3, text: 'Label 3'}]   // This "works," but probably isn't what you want
            lines: [{value: 'd', text: 'Label d'}]   // This does not work as of 56d9d450f1
        }
    }
});