qqplot_by_group

qqplot_by_group <- function(pval, group, ...) {
  n <- length(pval)
  pexp <- rank(pval) / n
  df <- data.frame(p.val = pval, grp = group) %>% group_by(grp) %>% mutate(p.exp = rank(p.val) / (n() + 1)) %>% ungroup()
  p <- ggplot(df) + 
    geom_point(aes(x = -log10(p.exp), y = -log10(p.val), color = grp), ...) + 
    geom_hline(yintercept = -log10(0.05 / n)) + 
    geom_abline(slope = 1, intercept = 0, linetype = 2)
  return(p)
}