Cosmetic bug with vertical focus line when Y axis is hidden and there are null points (line chart)

Reproducible on http://c3js.org/samples/tooltip_format.html, just add `show: false` to `axis.y` and make `data.columns` look like this, for example:
```js
columns: [
['data1', 30000, 20000, null, 40000, 15000, 250000],
['data2', 100, 200, null, 40, 150, 250]
],
```

Normally, hovering over points of a line chart shows a vertical focus line and a tooltip, and when hovering over an empty area (where points are null) there's no tooltip (which I guess is expected), but the vertical line gets shown on the very left of the chart. Not even on x=0, on left: 0px.

Reproduced in chromium on c3 0.4.15 and d3 3.5.17, also on whatever versions http://c3js.org/samples/tooltip_format.html uses at the moment.