arrows 函数用来在一张图表上添加箭头，只需要分别指定起始坐标和终止坐标，就可以添加箭头了，还可以通过一些属性对箭头的形状，大小进行调整
基本用法：
xo, yo 指定起始点的x和y坐标，x1, y1 指定终止点的x和y坐标， 代码示例如下：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4)
效果如下：
x0, y0,x1,y1 支持一次设置多个值，同时画多个箭头，示例代码如下：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
arrows(x0 = c(1, 1), y0 = c(1, 2), x1 = c(4, 4), y1 = c(4, 5))
效果如下：
参数调整：
length ： 该参数一次只能设置一个值，默认值为0.25， 为了调整不同箭头的大小，建议分别设置，用法如下：
par(mfrow = c(1,3))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "length = 0.1")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 0.1)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "length = 0.5")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 0.5)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "length = 1")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 1)
效果如下：
code : 调整箭头的类型，一共有1,2,3,4 共四种类型，该参数一次只能设置一个值，四种类型具体可以看下面的效果图
代码如下：
par(mfrow = c(1,3))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "code = 1")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, code = 1)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "code = 2")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, code = 2)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "code = 3")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, code = 3)
效果图如下：
code = 1 代表箭头由终止点指向起始点
code = 2 是默认值，箭头由起始点指向终止点
code = 3 代表在起始点和终止点两端都标上箭头
angle : 设置箭头的角度，默认值是45，该参数一次只能设置一个值，代码如下：
par(mfrow = c(1,3))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "angle = 15")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 0.5, angle = 15)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "angle = 45")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 0.5, angle = 45)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "angle = 60")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, length = 0.5, angle = 60)
效果图如下：
除了上面的针对arrows 的特殊参数之外，也支持一些通用的参数，col , lty ,lwd 等
代码如下：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
arrows(x0 = 1, y0 = 1, x1 = 4, y1 = 4, col = "red", lwd = 2, lty = 3)
效果图如下：
