index.html
<!DOCTYPE HTML>
<html>
<head>
<script src="//d3js.org/d3.v3.min.js"></script>
<style>
body {
margin: 0px;
padding: 0px;
}
</style>
</head>
<body>
<button type="button" id="clear">CLS</button>
<canvas id="myCanvas" width="400" height="400"></canvas>
<script>
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
var radius = 60;
context.beginPath();
context.arc(1/2 * centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'red';
context.fill();
context.lineWidth = 2;
context.strokeStyle = '#fff';
context.stroke();
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'green';
context.fill();
context.lineWidth = 2;
context.strokeStyle = '#fff';
context.stroke();
context.beginPath();
context.arc(3/2 * centerX, centerY, radius, 0, 2 * Math.PI, false);
context.fillStyle = 'blue';
context.fill();
context.lineWidth = 2;
context.strokeStyle = '#fff';
context.stroke();
d3.select('#clear').on('click', function() {
context.clearRect(0, 0, canvas.width, canvas.height);
}, false);
</script>
</body>
</html>