Optional Y Grid Lines

# grid_y_lines.js

var chart = c3.generate({ data: { columns: [ ['sample', 30, 200, 100, 400, 150, 250], ['sample2', 1300, 1200, 1100, 1400, 1500, 1250], ], axes: { sample2: 'y2' } }, axis: { y2: { show: true } }, grid: { y: { lines: [ {value: 50, text: 'Lable 50 for y'}, {value: 1300, text: 'Lable 1300 for y2', axis: 'y2', position: 'start'}, {value: 350, text: 'Lable 350 for y', position: 'middle'} ] } } });