Updating chart.js in ajax doesn't render the graph properly

I am trying to plot a line chart using chart.js with AJAX but the chart is not plotted properly when data comes from server. The chart is plotting fine on window onload event but with AJAX it failed to render properly. What could be the issue? Here is my code -

index.html:

<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>AJAX JSON Chart</title>
<script type="text/javascript" src="js/jquery-1.11.0.min.js"></script>
<script type="text/javascript" src="js/Chart.min.js"></script>
<script type="text/javascript">
var randomScalingFactor = function(){ return Math.round(Math.random()*100)};
var lineChartData = {
    labels : ["January","February","March","April","May","June","July"],
    datasets : [
        {
            label: "My First dataset",
            fillColor : "rgba(220,220,220,0.2)",
            strokeColor : "rgba(220,220,220,1)",
            pointColor : "rgba(220,220,220,1)",
            pointStrokeColor : "#fff",
            pointHighlightFill : "#fff",
            pointHighlightStroke : "rgba(220,220,220,1)",
            data : [randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor(),randomScalingFactor()]
        }
    ]

}

window.onload = function(){
var ctx = document.getElementById("canvas").getContext("2d");
window.myLine = new Chart(ctx).Line(lineChartData, {
    responsive: false
});
}

// ajax part of chart //
$(document).ready(function(e) {
    $("#frm1").submit(function(){
    var canvas = document.getElementById('canvas');
    ctx = canvas.getContext('2d');

    var btnchart = $("#btnchart").attr("name") + "=" + $("#btnchart").val();
        $.ajax({
            //beforeSend: function(){ sendRequest("canvas"); },
            cache: false,
            type: "POST",
            dataType: "json",
            url: "ajax/chart.php",
            data: $(this).serialize() + "&" + btnchart,
            success: function(data){
                //alert(data.labels);
                //alert(data.points);
                renderGraph(data.labels, data.points, ctx);
            },
        });
    return false;
    });
});

var renderGraph = function (labels, points, ctx) {

        var lineChartData = {
                        labels: "[" +  labels + "]",
                        datasets: [
                            {
                                label: "Recent Orders",
                                fillColor: "rgba(220,220,220,0.2)",
                                strokeColor: "rgba(220,220,220,1)",
                                pointColor: "rgba(220,220,220,1)",
                                pointStrokeColor: "#fff",
                                pointHighlightFill: "#fff",
                                pointHighlightStroke: "rgba(220,220,220,1)",
                                data: "[" +  points + "]", 
                            }
                        ]
                    };


        var myChart = new Chart(ctx)
                                .Line(lineChartData, {
                                        responsive: false,
                                        animation: false
                                        });
                                        myChart.update(lineChartData);

    }

function sendRequest(id)
{
    $("#"+id).html('<div style="width:100px; margin:0 auto;"><img id="loader-img" alt="" src="images/preloader.gif" width="100" height="100" /></div>');
}


</script>
</head>
<body>
<div style="width:30%">
    <div>
        <canvas id="canvas" height="450" width="960"></canvas>
    </div>
</div>
<form id="frm1" name="frm1" method="post" action="">
<input type="submit" name="btnchart" id="btnchart" value="Draw Chart" />
</form>
</body>
</html>

chart.php:

<?php
$labels = array('29 April 2015', '30 April 2015', '1 May 2015', '2 May 2015', '3 May 2015', '4 May 2015', '5 May 2015');
$points = array('100', '250', '10', '35', '73', '0', '25');
echo json_encode(array('labels' => $labels, 'points' => $points));
?>

On first time page loads, the output is: