adj 用来控制文字的对齐方式，取值范围为0到1，控制图片中x轴和y轴标签，标题，以及通过text 添加的文字的对齐方式
0表示左对齐，代码示例：
par(adj = 0)
plot(1:5, 1:5, type = "n", main = "title", xlab = "x", ylab = "y")
abline(h = 3, lty = 2)
abline(v = 3, lty = 2)
text(x = 3, y= 3, label = "text", cex = 2)
效果图如下：
0.5为默认值，表示居中对齐，代码示例：
par(adj = 0.5) plot(1:5, 1:5, type = "n", main = "title", xlab = "x", ylab = "y") abline(h = 3, lty = 2) abline(v = 3, lty = 2) text(x = 3, y= 3, label = "text", cex = 2)
效果图如下：
1代表右对齐，代码示例：
par(adj = 1) plot(1:5, 1:5, type = "n", main = "title", xlab = "x", ylab = "y") abline(h = 3, lty = 2) abline(v = 3, lty = 2) text(x = 3, y= 3, label = "text", cex = 2)
效果图如下:
