使用transform和arc方法，怎么绘制彩虹效果。

图6-30 彩虹效果<!DOCTYPE html><html><head>    <script type="text/javascript">        function draw(id) {            var canvas = document.getElementById('canvas');            if (canvas == null)                return false;            var context = canvas.getContext('2d');            /*定义颜色*/            var colors = ["#FF0000", "YELLOW", "#0D0", "#00F", "#C0C"];            /*定义线宽*/            context.lineWidth = 10;            context.transform(1, 0, 0, 1, 100, 0);            /*循环绘制椭圆*/            for (var i = 0; i < colors.length; i++) {                /*定义每次向下移动10个像素的变换矩阵*/                context.transform(1, 0, 0, 1, 0, 10);                /*设定颜色*/                context.strokeStyle = colors[i];                /*绘制圆弧*/                context.beginPath();                context.arc(30, 110, 100, 0, Math.PI, true);                context.stroke();            }        }    </script></head><body onload="draw();"><canvas id="canvas" width=260" height="180">    你的浏览器不支持Canvas</canvas>

使用transform和arc方法，怎么绘制彩虹效果。

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