library(tidyverse); library(ggthemes)
n = 34; delta = 0.8; sigma = 1; sims = 50000 # power ~= 0.9

tstats = replicate(sims, t.test(rnorm(n, delta, sigma))$statistic)
tstatsnull = replicate(sims, t.test(rnorm(n, 0, sigma))$statistic)

pvals = 1 - pf(tstats^2, df1 = 1, df2 = n - 1)
pvalsnull = 1 - pf(tstatsnull^2, df1 = 1, df2 = n - 1)

df = gather(tibble(pvals, pvalsnull), dist, pval)

ggplot(df, aes(pval, fill=dist)) + 
  geom_histogram(alpha=0.5, position="identity", binwidth=0.0025) + 
  scale_fill_colorblind(name="Rozkład (moc≈0,9)",labels=expression("H"[1],"H"[0]))+
  scale_x_continuous(name=expression("wartość "*italic("p"))) + 
  theme(legend.position="bottom") +
  geom_vline(xintercept=0.05) + coord_cartesian(xlim=c(0,0.07), ylim=c(0,500)) +
  scale_y_continuous(name="liczba symulacji", trans="sqrt")