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library(ggplot2)
a1 <- data.frame(No. = 1:4, ave1 = 1:4, ave2 = 2:5, ave3 = 3:6,
ave4 = 4:7, ave5 = 5:8, ave6 = 6:9)
colors <- c("ave1" = "red", "ave2" = "blue", "ave3" = "coral4", "ave4" = "orange",
"ave5" = "magenta", "ave6" = "darkolivegreen")
ggplot(data=a1, aes(x = No.)) +
geom_line(aes(y = ave1, colour= "ave1"),size=.8)+
geom_line(aes(y = ave2, colour= "ave2"), alpha=.2)+
geom_line(aes(y = ave3, colour= "ave3"), alpha=.2)+
geom_line(aes(y = ave4, colour= "ave4"), alpha=.2)+
geom_line(aes(y = ave5, colour= "ave5"), alpha=.2)+
geom_line(aes(y = ave6, colour= "ave6"),size=.8)+
xlab("age")+ylab("height")+
theme(legend.position="bottom") +
scale_color_manual(values = colors)

Created on 2020-05-21 by the reprex package (v0.3.0)