Date format X axis blank

Hello,

I try to display 3 areas and the time on X axis as Year-Month-Day Hour-Minute-Second. I did use the following code but nothing is displayed.

What am I doing wrong ?

Thank you

```
var chart = c3.generate({
data: {
x: 'x',
columns: [
['x', '2017-01-01 12:10:00', '2017-01-02 05:00:00', '2017-01-03 03:00:00', '2017-01-04 03:00:00',
'2017-01-05 03:00:00', '2017-01-06 03:00:00'],
['data1', 0.5, 0.5, null, null, null, null],
['data2', null, 0.5, 0.5, 0.5, 0.5, null],
['data3', null, null, null, null, 0.5, 0.5],

],
},
axis: {
x: {
type: 'timeseries',
tick: {
count: 4,
format: '%Y-%m-%d %H:%M:%S'
}
}
}
});

```