<canvas id="a_canvas" width="300" height="300"></canvas>
<script type="text/javascript">

var canvas = document.getElementById("a_canvas");
var ctx=canvas.getContext("2d");

function rk(nF, nT)
{
	var f=nT - nF +1;
	return Math.floor(Math.random()*f)+nF;
}

for(var j=0; j<10; j++){
	ctx.beginPath();
	
	var n=rk(2, 7);
	ctx.moveTo(rk(0, canvas.width), rk(0, canvas.height));
	for(var i=0; i<n; i++){
		ctx.lineTo(rk(0, canvas.width), rk(0, canvas.height));
	}

	ctx.closePath();

	ctx.fillStyle= "#"+rk(0, 0xFFFFFF), toString(16);
	ctx.fill();
}

</script>