pvalue plot vs uniform; FDR

qqunif  = function(p,BH=T,...)
{
  nn = length(p)
  xx =  -log10((1:nn)/(nn+1))
  plot( xx,  -sort(log10(p)),
     xlab=expression(Expected~~-log[10](italic(p))),
        ylab=expression(Observed~~-log[10](italic(p))),
       ... )
  abline(0,1,col='gray')
  if(BH)
    {
      lines( xx, -log10(0.05*(1:nn)/(nn+1)), col='red')
      lines( xx, -log10(0.10*(1:nn)/(nn+1)), col='orange')
      lines( xx, -log10(0.25*(1:nn)/(nn+1)), col='yellow')
      legend('bottomright', c("FDR = 0.05","FDR = 0.10","FDR = 0.25"),
             col=c('red','orange','yellow'),lty=1, cex=1.4)
    }
}