How to format my json data for stack column chart in HighCharts
===

Here is my json data


    var data = [
        {"unit":"a", "status":"Stopped / Idle", "val":21.2022222222222222},
        {"unit":"a", "status":"Working", "val":53.3066666666666667},
        {"unit":"a", "status":"Headland Turning", "val":0.04694444444444444444},
        {"unit":"a", "status":"Transport", "val":5.1425000000000000},
        {"unit":"b", "status":"Stopped / Idle", "val":334.7358333333333333},
        {"unit":"b", "status":"Working", "val":212.6386111111111111},
        {"unit":"b", "status":"Headland Turning", "val":26.2955555555555556},
        {"unit":"b", "status":"Transport", "val":0.00444444444444444444}
    ];


`unit` is the category

Thank you. 
