Multiple categories and labels for chart

I'm trying to build a stacked bar chart for one of the requirement. The chart should look like as follows:

The problem here is kind of multiple categories.

The first level of categorization is Proj1 and Proj2 and the next level is R1.0 and R2.0. I've tried the following:

``` javascript
var chart = c3.generate({
bindto: '#chart',
data: {
columns: [
['Proj1Item1', 30, 200, 100],
['Proj1Item2', 50, 20, 10],
['Proj2Item1', 20, 120, 50],
['Proj2Item2', 30, 20, 80],
],
type: 'bar',
groups: [
['Proj1Item1', 'Proj1Item2'],
['Proj2Item1', 'Proj2Item2']
]

},
axis: {
x: {
type: 'category',
categories: ['R1.0', 'R2.0', 'R3.0']
}
}
});
```

Is it not possible with C3 to achieve this? I tried googling around and not able to find the solution for it.

Start with the supplied c3.generate configuration and reproduce the grouped series with categorical x-axis behavior. Determine whether two category levels and the requested labels are supported; done means providing a reproducible answer or a clearly scoped C3 change, with the relevant code and tests identified after locating the chart implementation.

Written by the indexing model from the issue text.