Morris.js donut chart not displaying chart when getting data through Ajax from php script

I am working on displaying data through a morris.js donut chart. These data come from a MySQL database through PHP and ajax.

The typical example where you type the data to be displayed right in the code is working fine

Morris.Donut({
            element: 'donut-example',
            data: [
              { label: "Download Sales", value: 12 },
              { label: "In-Store Sales", value: 30 },
              { label: "Mail-Order Sales", value: 20 }]
        });

however, when i call the data from the database, the chart is not displayed. Moreover, firebug does not show any errors.

This is the output i get from PHP:

[{label: "4 pairs",value: "9"},{label: "3 pairs",value: "9"},{label: "2 pairs",value: "6"},{label: "5 pairs",value: "3"},{label: "6 pairs",value: "2"},{label: "1 pair",value: "2"}]

Actually, I tried this output by copying inside the Morris.Donut(); function and it worked fine!

And this is the Ajax code (which doesn't work at all):

$.ajax({
    url: "ajax/modas_pares.php",
    type: "post",
    data: {lotto: 'the_lotto', limit: 20},
    success : function (resp){
        //alert(resp);
        Morris.Donut({
            element: 'donut-example',
            data: resp
        });
    },
    error: function(jqXHR, textStatus, ex) {
      console.log(textStatus + "," + ex + "," + jqXHR.responseText);
    }
});

However, if i have the following code:

$.ajax({
            url: "ajax/modas_pares.php",
            cache: false,
            type: "post",
            data: {lotto: 'the_lotto', limit: 20},
            //dataType: "json",
            timeout:3000,
            success : function (resp){
                //var column_data = $.parseJSON(resp);
                //alert(resp);
                Morris.Donut({
                    element: 'donut-example',
                    data: [{label: "3 pairs",value: "6"},{label: "4 pairs",value: "5"},{label: "2 pairs",value: "5"},{label: "5 pairs",value: "3"},{label: "1 pair",value: "1"},{label: "6 pairs",value: "1"}]
                });
            },
            error: function(jqXHR, textStatus, ex) {
              console.log(textStatus + "," + ex + "," + jqXHR.responseText);
            }
        });

It does work, fine!!!

What i don't get is that, if i tried passing the code through a variable, it does not work!! :

$string='[{label: "4 pairs",value: "9"},{label: "3 pairs",value: "9"},{label: "2 pairs",value: "6"},{label: "5 pairs",value: "3"},{label: "6 pairs",value: "2"},{label: "1 pair",value: "2"}]';
?>

And

$.ajax({
            url: "ajax/modas_pares.php",
            cache: false,
            type: "post",
            data: {lotto: 'the_lotto', limit: 20},
            //dataType: "json",
            timeout:3000,
            success : function (resp){
                //var column_data = $.parseJSON(resp);
                //alert(resp);
                Morris.Donut({
                    element: 'donut-example',
                    data: '<?php echo $string ?>'
                });
            },
            error: function(jqXHR, textStatus, ex) {
              console.log(textStatus + "," + ex + "," + jqXHR.responseText);
            }
        });

It doesn't work :( if i get it either from a php variable or from a php script through ajax :/

What am i missing? How can i fix this? Why?????

The error comes right where you set up the array. In the PHP section you should make the array as follows:

$resp = array(
      array('label' => '5 pairs','value' => 4),
      array('label' => '3 pairs','value' => 8)
);
echo json_encode( $resp );

Then you can use:

$.ajax({
        url: "ajax/modas_pares.php",
        type: "post",
        data: {loteria: 'melate', limit: 20},
        dataType: "json",
        timeout:3000,
        success : function (resp){
            Morris.Donut({
                element: 'donut-example',
                data: resp
            });
        },
        error: function(jqXHR, textStatus, ex) {
          console.log(textStatus + "," + ex + "," + jqXHR.responseText);
        }
    });