折线：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>用canvas绘制折线图</title>
</head>
<body>
<canvas id="cv"></canvas>
</body>
</html>
<script>
var cv = document.getElementById("cv");
cv.width = 600;
cv.height = 400;
cv.style.border = "1px solid red";
var ctx = cv.getContext("2d");
var data2 = [.3, .1, .2, .4, .2, .7, .3, .9];
var data3 = [3, 12, 14, 17, 29, 33, 40, 52];
getBrokenLine(data2, "#f00");
getBrokenLine(data3, "#0f0");
//封装一个折线图的函数
function getBrokenLine(data, color) {
var maxNum = Math.max.apply(null, data); //求数组中的最大值
var padding = 20, //边距
x0 = padding, //原点x轴坐标
y0 = cv.height - padding, //原点y轴坐标
xArrow_x = padding, //x轴箭头处坐标x
xArrow_y = padding, //x轴箭头处坐标y
yArrow_x = cv.width - padding, //y轴箭头处坐标x
yArrow_y = cv.height - padding, //y轴箭头处坐标y
arrowWidth = 10, //箭头的宽度
xLength = cv.width - 2*padding - arrowWidth, //x轴的长度
yLength = cv.height - 2*padding - arrowWidth, //y轴的长度
pointsWidth = xLength/(data.length + 1); //折线上每个点之间的距离
ctx.beginPath();//控制绘制的折线不受坐标轴样式属性的影响
//绘制x轴
ctx.moveTo(x0, y0);
ctx.lineTo(xArrow_x, xArrow_y);
ctx.moveTo(xArrow_x, xArrow_y);
ctx.lineTo(xArrow_x - arrowWidth, xArrow_y + arrowWidth);
ctx.moveTo(xArrow_x, xArrow_y);
ctx.lineTo(xArrow_x + arrowWidth, xArrow_y + arrowWidth);
//绘制y轴
ctx.moveTo(x0, y0);
ctx.lineTo(yArrow_x, yArrow_y);
ctx.moveTo(yArrow_x, yArrow_y);
ctx.lineTo(yArrow_x - arrowWidth, yArrow_y - arrowWidth);
ctx.moveTo(yArrow_x, yArrow_y);
ctx.lineTo(yArrow_x - arrowWidth, yArrow_y + arrowWidth);
ctx.strokeStyle = "#000";
//中断（坐标轴和折线的）连接
ctx.stroke();
ctx.beginPath();
//绘制折线
for (var i = 0; i < data.length; i++) {
var pointX = padding + (i + 1) * pointsWidth;
var pointY = padding + arrowWidth + (1 - data[i]/maxNum) * yLength;
ctx.lineTo(pointX, pointY);
}
ctx.strokeStyle = color;
ctx.stroke();
}
</script>
画直线：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>用面向对象的思想 封装 在canvas绘制直线的函数</title>
</head>
<body>
<canvas id="cv"></canvas>
</body>
</html>
<script>
var cv = document.getElementById("cv");
cv.width = 600;
cv.height = 300;
cv.style.border = "1px solid red";
var ctx = cv.getContext("2d");
//面向对象编程
//1 创建构造函数
//2 构造函数的原型设置
//3 调用的时候，通过 new+构造函数 来创建一个对象（实例）
//构造绘制直线的函数
function drawLine(parameters) {
this.init(parameters);
}
//替换原型对象实现继承
drawLine.prototype = {
constructor: drawLine,
init: function (parameters) {
this.ctx = parameters.ctx;
this.startX = parameters.points[0];
this.startY = parameters.points[1];
this.endX = parameters.points[2];
this.endY = parameters.points[3];
//以下3个属性，可以不设置，用短路运算实现添加默认属性值
this.lineWidth = parameters.lineWidth || 1;
this.lineDash = parameters.lineDash || [];
this.strokeStyle = parameters.strokeStyle || "#000";
},
//原型中，一般用来储存对象的方法或者共有的属性
stroke: function () {
this.ctx.beginPath();
this.ctx.moveTo(this.startX, this.startY);
this.ctx.lineTo(this.endX, this.endY);
this.ctx.lineWidth = this.lineWidth;
this.ctx.setLineDash(this.lineDash);
this.ctx.strokeStyle = this.strokeStyle;
this.ctx.stroke();
}
};
//调用构造函数，传入参数
var line = new drawLine({
ctx: ctx,
points: [100, 100, 300, 100],
lineDash: [4, 2],
strokeStyle: "red"
});
line.stroke();
var line2 = new drawLine({
ctx: ctx,
points: [100, 200, 300, 200],
lineWidth: 6
});
line2.stroke();
</script>