JSON returns STRING instead of ARRAY

The following code is returning an array as a string and therefore I can not distribute the data to popular flot chart.

O

PHP code:

<?php

include_once("config.php");

$viagemid = $_POST['id'];
$results_array = array();

$sql = "SELECT adiantamento AS 'Antecipado' FROM viagem WHERE viagemid = $viagemid";
$query = $mysqli->query($sql);
while($row =mysqli_fetch_assoc($query)){

    array_push($results_array, $row);
}

$sql = "SELECT SUM(vl_desp) as 'Despesas' FROM despesas WHERE viagemid = $viagemid";
$query = $mysqli->query($sql);
while($row =mysqli_fetch_assoc($query)){

    array_push($results_array, $row);
}


$sql = "SELECT SUM(venda.vl_venda * venda.qtd) AS 'Venda SP' FROM venda WHERE venda.viagemid = $viagemid AND venda.ufid = 1";
$query = $mysqli->query($sql);
while($row =mysqli_fetch_assoc($query)){

    array_push($results_array, $row);
}

$sql = "SELECT SUM(venda.vl_venda * venda.qtd) AS 'Venda local' FROM venda WHERE venda.viagemid = $viagemid AND venda.ufid = 2";
$query = $mysqli->query($sql);
while($row =mysqli_fetch_assoc($query)){

    array_push($results_array, $row);
}



$string = "[[0,".$results_array[0]['Antecipado']."],[1,".$results_array[1]['Despesas']."],[2,".$results_array[2]['Venda SP']."],[3,".$results_array[3]['Venda local']."]]";


var_dump(json_decode(serialize($results_array), true));

?>

The JS:

    $(function() {

        $.post('chart_data.php', {id:<?php echo $viagemid; ?>}, function(data) {

            alert(data);
            var d1 = [[0,data],[1,data],[2,data],[3,data]];

            alert(d1);

        var barOptions = {
            series: {
                bars: {
                    show: true,
                    barWidth: 0.6,
                    fill: true,
                    align:'center',
                    fillColor: {
                        colors: [{
                            opacity: 0.8
                        }, {
                            opacity: 0.8
                        }]
                    }
                }
            },
            xaxis: {
                tickDecimals: 0,
                ticks:[[0,'Adiantamento'],[1,'Despesas'],[2,'Venda SP'],[3,'Venda local']]
            },
            colors: ["#ffbb00"], 
            grid: {
                color: "#999999",
                hoverable: true,
                clickable: true,
                tickColor: "#D4D4D4",
                borderWidth:0
            },
            legend: {
                show: false
            },
            tooltip: true,
            tooltipOpts: {
                content: "%x: %y"
            }

        };
        var barData = {
            label: "bar",
            data: d1
        };
        $.plot($("#flot-bar-chart"), [barData], barOptions);

    });

        //return false;
});