axis函数用来在一张图表上添加轴线，区别于传统的x轴和y轴，axis 允许在上，下，左， 右4个方向添加轴线
以x轴为例，一条轴线包含3个元素，水平的一条横线，叫做axis line , 刻度线， 叫做tick line, 对应的标签 labels
基本用法：
通过side 参数设置需要添加的轴线的方向，从下边开始，沿逆时针方向，数字为1到4
代码示例：
par(oma = c(1, 1, 1, 1), mfrow = c(1, 4))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
text(x = 3, y = 3, labels = "side = 1", cex = 2)
box()
axis(side = 1)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
text(x = 3, y = 3, labels = "side = 2", cex = 2)
box()
axis(side = 2)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
text(x = 3, y = 3, labels = "side = 3", cex = 2)
box()
axis(side = 3)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
text(x = 3, y = 3, labels = "side = 4", cex = 2)
box()
axis(side = 4)
效果图如下：
参数设置：
at : 需要添加刻度的数值，默认会根据变量的取值范围计算几个合适的刻度，也可以手工指定
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, at = c(0, 2, 4, 6))
效果图如下：
lables : 指定在刻度上需要标记的内容，默认就是刻度对应的值
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, at = c(0, 2, 4, 6), labels = paste(c(0, 2, 4, 6), "A", sep = ""))
效果图如下：
tick : 逻辑值，是否显示轴线，包括刻度线和对应的轴线， FALSE 表示不显示
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
axis(side = 1, tick = F)
效果图：
line : 轴线的位置
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1)
效果图如下：
lwd : 设置 axis line 和 tick line 的宽度
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1, lwd = 2)
效果图如下：
lwd.tick : 设置tick line的宽度
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1, lwd = 1, lwd.tick = 2)
效果图如下：
lty : 设置axis line 和tick line的线条类型
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1, lty = 3)
效果图如下：
col : 设置axis line 和 tick.line 的线条颜色
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1, col = "blue")
效果图如下：
col.ticks : 设置tick line的颜色
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, line = 1, col = "blue", col.ticks = "red")
效果图如下：
pos : 对轴线的位置进行调整，当pos 设置了对应的值之后会覆盖line 参数的值
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", axes = F)
box()
axis(side = 1, pos = 1)
效果图如下：
