我无法让 Paper.js 绘制矩形。我已经按照教程进行了操作，但似乎没有任何效果。你能告诉我下面的代码有什么问题吗？
<!DOCTYPE html>
<html>
<head>
<!--load paper.js-->
<script type="text/javascript" src="http://localhost/node_modules/paper/dist/paper-core.js"></script>
<!--drawing script-->
<script type="text/javascript" canvas="canvas">
window.onload = function() {
var canvas = document.getElementById("canvas");
paper.setup(canvas);
var meterStart = new paper.Point(5,5);
var meterSize = new paper.Size(40, 720);
var meter = new paper.Rectangle(meterStart, meterSize);
meter.strokeColor = 'black';
paper.view.draw();
}
</script>
</head>
<body>
<canvas id="canvas" height="750px" width="600px"></canvas>
</body>
</html>