Y axis ticks not visible if all data values are 0

Hello,

I've noticed behavior I do not understand. If I have a chart where all data is simply '0's like so:

```
var chart = c3.generate({
data: {
columns: [
['data1', 0, 0, 0]
]
}
});
```

Then Y axis ticks are not visible at all, even for the 0. I can see points on Y=0 axis but it's strange that one of the axis seems to not have any units. When I change it for all '1's:

```
var chart = c3.generate({
data: {
columns: [
['data1', 1, 1, 1]
]
}
});
```

Then some Y ticks are created and displayed, and the points are actually connected. '0' is a magic number which brakes visibility of Y axis ticks.