I wanted to construct a bar chart using chart.js. I typed this code:

```
<!DOCTYPE html>
<html>
<head>
    <title>Bar chart</title>
    <script src="Chart.js"></script>
</head>
<body>
    <canvas id="canvas" width="256" height="256"></canvas>
    <script>
        var my = new Chart(chr).Bar(data);
        var chr = document.getElementById("canvas");
        var ctx = canvas.getContext("2d");
        var data = {
            dataset: [
                {
                    label: "my first dataset",
                    fillColor: "blue",
                    strokeColor: "green",
                    data: [65, 53, 80, 83, 55, 45]
                }
            ]
        };
        var myfirstChart = new Chart(chr).Bar(data);
    </script>
    console.log(ctx);
</body>

</html>

```

...and it says there is cannot "read property length undefined".

What is the error and how to correct it?