Paired Stacked Multibar

Question

Starting form the example on Grouped / Stacked Multi-Bar I draw this graph using nvd3.js:

When I click on the Stacked control, the second series is not drawn from the start of the axis:

I dont understand why is that. Here is a jsfiddle example of my code: http://jsfiddle.net/58rw7/1/

My data looks like this:

[
    {
        "key": "a - BAR_FM - kWh - F1",
        "values": [
            {
                "x": "0_0",
                "y": 50,
                "label": "1/1/2014"
            },
            {
                "x": "0_1",
                "y": 108,
                "label": "2/1/2014"
            },
            {
                "x": "0_2",
                "y": 109,
                "label": "3/1/2014"
            }
        ]
    },
    {
        "key": "a - CONDIZIONAMENTO_IDRICO - kWh - F1",
        "values": [
            {
                "x": "1_0",
                "y": 186,
                "label": "1/1/2014"
            },
            {
                "x": "1_1",
                "y": 186,
                "label": "2/1/2014"
            },
            {
                "x": "1_2",
                "y": 184,
                "label": "3/1/2014"
            }
        ]
    }
]

Why the second series is not drawn from the start of the axis? How can I do that?