x<-seq(0,3,by=0.001)
y1<-1+sqrt(x)
y2<-1-sqrt(x)
y3<-x
plot(x,y1,type="l",ylim = c(-1,3),ylab = "y",col="blue")
lines(x,y2,type = "l",col="red")
lines(x,y3,type = "l",col="green")
如果需要图例，可以留言，生成图例

x<-seq(0,3,by=0.001)
y1<-1+sqrt(x)
y2<-1-sqrt(x)
y3<-x
plot(x,y1,type="l",ylim = c(-1,3),ylab = "y",col="blue")
lines(x,y2,type = "l",col="red")
lines(x,y3,type = "l",col="green")
生成的函数图如附件所示

多谢朋友！