How can I change the sort order in charts

I create a Flexmonster with JSON data, like:

var pivot = new Flexmonster({
    container: "pivot",
    report: {
        dataSource: {
            dataSourceType: "json",
            filename: "/api/v1/stats/"
        }
    },
    reportcomplete: function() {
        pivot.off("reportcomplete");
        createChart();
    }
})

And then I render a Highcharts Column-Chart like:

function createChart() {
    var render = function(data) {
        Highcharts.chart('pivot_chart', data);
    }
    pivot.highcharts.getData(
        {
            type: "column",
            "slice": {
                "rows": [
                    {
                        "uniqueName": "total_salary",
                    }
                ],
                "columns": [
                    {
                        "uniqueName": "[Measures]"
                    },
                    {
                        "uniqueName": "status"
                    }
                ],
                "measures": [
                    {
                        "uniqueName": "number_of_persons",
                        "aggregation": "sum"
                    }
                ]
            }
        },
        render,  // callbackHandler
        render   // updateHandler
    );
}

My total_salary values look like: "< 30'000", "30'000 - 40'000", "60'000 - 70'000" but they are not ordered like this in the chart. I tried to set the sortOrder like:

"rows": [
    {
        "uniqueName": "total_salary",
        "sortOrder": ["< 30'000", "30'000 - 40'000", "60'000 - 70'000"]
    }
],

But it's not working. Can somebody explain me how to do this? Thanks a lot!