subchart does not display data for scatter plot

If you add a subchart to a scatter plot, the data points will not be visible in the subchart but it will otherwise work just fine.

I tested it by simply changing the type of http://c3js.org/samples/options_subchart.html

```
var chart = c3.generate({
data: {
columns: [
['sample', 30, 200, 100, 400, 150, 250]
],
type: 'scatter'
},
subchart: {
show: true
}
});
```

Reproduce the issue by changing the options_subchart.html example to use scatter data with subchart.show enabled. Trace the subchart rendering path and compare it with the main scatter plot rendering. Done means the scatter data points appear in the subchart without breaking the existing example behavior.

Written by the indexing model from the issue text.