library(ggplot2)


X <- seq(-5, 5, length.out = 1000)
dfn <- dt(X, df = 1)
norm <- dnorm(X)

temp <- data.frame(X, dfn, norm)
Table <- melt(temp, id.vars = "X", variable.name = "Types", value.name = "Y")

ggplot(data = Table, aes(x = X, y = Y, col = Types, linetype = Types)) + 
       geom_line(size = 1) + 
       ggtitle("不同自由度下的t分布以及标准正态分布") + 
       theme(plot.title = element_text(hjust = 0.5)) + 
       theme_bw()