board.onmousedown = function(e) {
    var x = e.pageX - this.offsetLeft;
    var y = e.pageY - this.offsetTop;
    ctx.fillStyle = "#000000"
    ctx.beginPath();
    ctx.arc(x, y, 10, 0 , 2 * Math.PI, false);
    ctx.fill();
    ctx.closePath();
    ctx.fillText(++node_id, x+10, y-10);
    node_set.push({id: node_id, "x": x, "y": y});
}