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