parameter_grid <- expand.grid(n = c(32, 64),
                              d = seq(0.05, 2, by = 0.05),
                              sd = seq(1, 2, by = 0.2))

parameter_grid$power <- power.t.test(n = parameter_grid$n, delta = parameter_grid$d, sd = parameter_grid$sd)$power

library(tidyverse)
parameter_grid$n <- paste(parameter_grid$n, "pro group")
ggplot(parameter_grid,
       aes(x = d,
           y = power,
           colour = factor(sd))) +
  geom_line() +
  xlab("mean difference in population") +
  scale_colour_brewer(name = "Population SD",
                      palette = "Dark2") +
  facet_wrap(~ n) +
  theme_bw(12) +
  theme(legend.position = "bottom")