text函数用来在一张图表上添加文字，只需要指定对应的x和y坐标，以及需要添加的文字内容就可以了
基本用法：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = 3, y = 3, labels = "text")
效果图如下：
支持同时创建多个text标签
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = c(3, 3), y = c(3, 5), labels = c("text", "text"))
效果图如下：
参数调整：
col : 设置文字的颜色
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = c(3, 3), y = c(3, 5), labels = c("text", "text"), col = c("red", "blue"))
效果图如下：
cex : 设置文字的大小
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = c(3, 3), y = c(3, 5), labels = c("text", "text"), cex = c( 0.5 , 2 ))
效果图如下：
sort : 对文件进行旋转
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = 3, y = 3, labels = "text", srt = 45)
效果图如下：
adj : 调整文字的位置，一个值时调整的是x轴的位置，如果为两个值时，第一个调整的是x轴的位置，第二个调整的是y轴的位置，可选范围为[0, 1]
对x轴进行调整，代码示例：
par(mfrow = c(1,3))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 0")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = 0)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 0.5")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = 0.5)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 1")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = 1)
x轴调整的效果图如下：
对y轴进行调整，代码示例：
par(mfrow = c(1,3))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 0")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = c(0.5, 0))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 0.5")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = c(0.5, 0.5))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 1")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", adj = c(0.5, 1))
y轴调整的效果图如下：
pos: 也是对文字的位置进行调整，不能和adj参数同时使用, 可选值为1， 2， 3, 4， 分别对应下， 上， 左， 右4个方向
代码示例：
par(mfrow = c(1, 4))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "pos = 1")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", pos = 1)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "pos = 2")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", pos = 2)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "pos = 3")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", pos = 3)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "adj = 4")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", pos = 4)
效果图如下：
offset : 只有和和pos 函数搭配起来才会产生作用，对文字的文字进行调整
代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "pos = 1")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", pos = 1, offset = 1, col = "red")
text(x = 3, y = 3, labels = "text", pos = 1, offset = -1, col = "blue")
效果图如下：
font: 设置文字的格式，1是默认值，就是普通的文字，2代表加粗，3代表斜体， 4代表加粗+斜体， 5只有用来ADOBE的设备上时，才有用
代码示例：
par(mfrow = c(1, 5))
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "font = 1")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", font = 1)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "font = 2")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", font = 2)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "font = 3")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", font = 3)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "font = 4")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", font = 4)
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n", main = "font = 5")
abline(h = 3, v = 3 , col = "gray", lty = 3)
text(x = 3, y = 3, labels = "text", font = 5)
效果图如下：
text函数的常见用法和参数就参考上面的例子就够了，当我们想打印数学表达式或者一些特殊符号在图片上是，就需要对lables 参数进行特别设置
对于数据表达式，使用expression 函数将其转化成表达式后，在显示出来会更好
打印一个开平方的表达式，代码示例：
plot(1:5, 1:5, xlim = c(0,6), ylim = c (0,6), type = "n")
text(x = 3, y = 3, labels = expression(sqrt(x)))
效果图如下：
对于表达式的打印，具体的可以参考 plotmath 函数的帮助文档