<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'grey';
pincel.fillRect(0, 0, 600, 400);
function desenhaCirculo(evento) {
var x = evento.pageX - tela.offsetLeft;
var y = evento.pageY - tela.offsetTop;
var key = evento.shiftKey;
pincel.fillStyle = 'blue';
pincel.beginPath();
if (key){pincel.arc(x, y, 20, 0, 2 * 3.14);}
else{pincel.arc(x, y, 10, 0, 2 * 3.14);}
pincel.fill();
console.log(x + ',' + y);
}
tela.onclick = desenhaCirculo;
</script>
