Style for Region

# style_region.js
var chart = c3.generate({
    data: {
        columns: [
            ['sample', 30, 200, 100, 400, 150, 250]
        ]
    },
    regions: [
        {start:0, end:1},
        {start:2, end:4, class:'foo'}
    ]
});

# style_region.css

.c3-region-0 {
  fill: red;
}
.c3-region.foo {
  fill: green;
}