library(ggplot2)
library(tibble)

x_plt <- seq(0, pi/2, len=100)
tb <- tibble(
  x_val = rep(x_plt, 2), 
  y_val = c(sin(x_plt), sin(2*x_plt)), 
  fun = rep(c("sin(x)", "sin(2x)"), each=length(x_plt)))

plt <- ggplot(tb, aes(x_val, y_val, color=fun)) + 
    xlab("x") +
    ylab("y") +
    labs(color = "function") +
    scale_x_continuous(breaks = pi/c(Inf, 6, 4, 3, 2),
                       labels = c("0", "pi/6", "pi/4", "pi/3", "pi/2")) +
    scale_y_continuous(breaks = sqrt(0:4/4),
                       labels = c("0", "1/2", "sqrt(2)/2", "sqrt(3)/2", "1")) +
    geom_line()

print(plt)