Chart js pie chart not showing dynamic data using JSON

Right now you are trying to create chart without waiting your post call return data. Move your chart creation code into $.post callback like this:

$.post("<?php echo base_url(). ;?>",
function(data){

    var obj = JSON.parse(data);
    $.each(obj,function(i,item){
        numpassed.push(item.passed);
        numfailed.push(item.failed);
    });

    var doughnutData = [
        {
            value: numpassed,
            color:"#F7464A"
        },
        {
            value : numfailed,
            color : "#46BFBD"
        },

    ];

    new Chart(document.getElementById("doughnut").getContext("2d")).Doughnut(doughnutData);
});