画布无法在IE7-10上运行

i have a page to draw some graphs on canvas.

I use Ajax to take datas in Json file, put them in the array and draw them in canvas.

It works on Chrome, Firefox, IE 11 but not working on lower IE than 11, even if I have excanvas.js included.

I have no idea where the problem can be.

My Ajax ->

$.ajax({
    url: GlobalUrl,
    cache: false,
    //force to handle it as text
    dataType: "text",
    success: function (data) {

        //data downloaded so we call parseJSON function 
        //and pass downloaded data
        var json = $.parseJSON(data);......
        Some operations with data....

        Draw function.

I think the problem might be here but not sure. Thanks for answers