connectNull rendering issue with multiple columns of data

I'm using line.connectNull on a chart with multiple grouped columns, and setting `line.connectNull` to `true` causes a rendering issue on x values where both columns intersect:


When `line.connectNull` is set to `false`, the chart renders as expected:


And here is code that reproduces the bug:

```
chart = c3.generate({
data: {
json: {
'series': [null, null, 200, 500],
'series1': [200, 200, null, 440],
'series2': [200, 500, null, 400],
'series3': [null, null, 350, 500]
},
groups: [
[
'series',
'series1',
'series2',
'series3'
]
],
types: {
'series': 'area',
'series1': 'area',
'series2': 'area',
'series3': 'area'
}
},
line: {
connectNull: true
},
axis: {
x: {
categories: [
'www.site1.com',
'www.site2.com',
'www.site3.com',
'www.site4.com'
],
type: 'category'
}
}
});
```