下面的代码可以画出一张图，包含四个子图，如上图
怎么在这张图的最下方标明实线、虚线等
par(mfrow = c(2, 2))
r1<-read.table("F:/RLCUSUM1/1.txt", head=TRUE)
plot(log(x1)~delta,data=r1,type="l",ylab= "log(ARL)",lty=1,main="m=0",
xlab=expression(delta))
lines(log(x2)~delta,data=r1,type="l",ylab= "log(ARL)",lty=2,
xlab=expression(delta))
lines(log(x3)~delta,data=r1,type="l",ylab= "log(ARL)",lty=3,
xlab=expression(delta),lwd=1.8)
lines(log(x4)~delta,data=r1,type="l",ylab= "log(ARL)",lty=4,
xlab=expression(delta),lwd=1.8)


r2<-read.table("F:/RLCUSUM1/2.txt", head=TRUE)
plot(log(x1)~delta,data=r2,type="l",ylab= "log(ARL)",lty=1,main="m=1",
xlab=expression(delta),ylim=c(0.5,5.5))
lines(log(x2)~delta,data=r2,type="l",ylab= "log(ARL)",lty=2,
xlab=expression(delta))
lines(log(x3)~delta,data=r2,type="l",ylab= "log(ARL)",lty=3,
xlab=expression(delta),lwd=1.8)
lines(log(x4)~delta,data=r2,type="l",ylab= "log(ARL)",lty=4,
xlab=expression(delta),lwd=1.8)

r3<-read.table("F:/RLCUSUM1/3.txt", head=TRUE)
plot(log(x1)~delta,data=r3,type="l",ylab= "log(ARL)",lty=1,main="m=2",
xlab=expression(delta),ylim=c(0.5,5.5))
lines(log(x2)~delta,data=r3,type="l",ylab= "log(ARL)",lty=2,
xlab=expression(delta))
lines(log(x3)~delta,data=r3,type="l",ylab= "log(ARL)",lty=3,
xlab=expression(delta),lwd=1.8)
lines(log(x4)~delta,data=r3,type="l",ylab= "log(ARL)",lty=4,
xlab=expression(delta),lwd=1.8)


r4<-read.table("F:/RLCUSUM1/4.txt", head=TRUE)
plot(log(x1)~delta,data=r4,type="l",ylab= "log(ARL)",lty=1,main="m=4",
xlab=expression(delta),ylim=c(0.5,5.5))
lines(log(x2)~delta,data=r4,type="l",ylab= "log(ARL)",lty=2,
xlab=expression(delta))
lines(log(x3)~delta,data=r4,type="l",ylab= "log(ARL)",lty=3,
xlab=expression(delta),lwd=1.8)
lines(log(x4)~delta,data=r4,type="l",ylab= "log(ARL)",lty=4,
xlab=expression(delta),lwd=1.8)