js绘制canvas图

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
  <head>
    <meta charset='utf-8'>
    <title>画图</title>
    <style>
      #divContainer{
        margin-top: 20px;
        text-align: center;
      }
      #cv{
        width: 300px;
        height: 200px;
        border-bottom: 2px solid #000;
        border-left: 2px solid #000;
      }
    </style>
  </head>
  <body>
  <%
  
  %>
    <div id="divContainer">
      铝锭价走势图<br/>
      <canvas id="cv">你的设备不支持图表数据显示</canvas>
    </div>
    <script>
      (function(){
        window.onload = function(){
          //数据源
          var dict = [
            {x: "2015-04-24", y: 13400},
            {x: "2015-04-25", y: 13380},
            {x: "2015-04-26", y: 13370},
            {x: "2015-04-27", y: 13370},
            {x: "2015-04-28", y: 13380}
          ]
          //数据源提取
          var len = dict.length;
          var xArr = [], yArr = [], tmp_yArr = [];
          for(var i=0; i<len; i++){
            xArr.push(i * 60);
            tmp_yArr.push(dict[i].y);
          }
          var tmp_minY = Math.min.apply(Math, tmp_yArr);//最小值
          var tmp_maxY = Math.max.apply(Math, tmp_yArr);//最大值
          if(tmp_maxY - tmp_minY <= 100){
            for(var i=0; i<len; i++){
              yArr.push(tmp_yArr[i] - tmp_minY + 50);//与最小的做比较
            }
          }
          else{//如果相差太大会导致图表不美观
            for(var i=0; i<len; i++){
              yArr.push(tmp_yArr[i] / 500);
            }
          }
          var minY = Math.min.apply(Math, yArr);
          var maxY = Math.max.apply(Math, yArr);
          //canvas 准备
          var canvas = document.getElementById("cv");//获取canvas画布
          var ctx = canvas.getContext("2d");
          //画折线
          for(var i=0 ;i<len; i++){
            var x = xArr[i];
            var y = maxY - yArr[i] + minY;
            if(i === 0){
              ctx .moveTo(x, y);
            }
            else{
              ctx .lineTo(x, y);
            }
          }
          ctx .stroke();
          //画点
          for(var i=0; i<len; i++){
            var x = xArr[i];
            var y = maxY - yArr[i] + minY;
            var xMemo = dict[i].x;
            var yMemo = "¥" + dict[i].y;
            ctx.beginPath();
            ctx.fillStyle = "#000";
            ctx.arc(x, y, 2, 0, 2*Math.PI);//画点
            ctx.fill();
            ctx.fillText(yMemo, x + 3, y - 10);
            ctx.fillText(xMemo, x + 3, canvas.height - 10, 40);//画文字
          }
        }
      })();
    </script>
  </body>
</html>
实现效果：
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