Bar charts fail with `axis.x.tick.values = []`

Go to the ["X Tick Values examples"](http://c3js.org/samples/axes_x_tick_values.html), and enter the following code:

``` js

var chart = c3.generate({
data: {
type: 'bar',
x: 'x',
columns: [
['x', '2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04', '2013-01-05', '2013-01-06', '2013-01-07', '2013-01-08', '2013-01-09', '2013-01-10', '2013-01-11', '2013-01-12'],
['sample', 30, 200, 100, 400, 150, 250, 30, 200, 100, 400, 150, 250]
]
},
axis: {
x: {
type: 'timeseries',
tick: {
// this also works for non timeseries data
values: []
}
}
}
});
```

This results in no data being displayed. I also get intermittent errors of `Invalid value for attribute d="MNaN,250...`

If you change the graph type to `'line'`, it works fine.

I ran into this while trying to remove ticks from my x axis.