The only way to get the alpha levels is to determine what they will be with p.adjust()
, since they will depend on the distribution of your unadjusted p values. The general steps you should follow will be:
- Add a column of adjusted p-values to your dataframe (
mydata$padj = p.adjust(mydata, method="BH")
, which is the same as FDR and saves a character). - Use
which
andmax
to determine your two alpha threshold (e.g.,max(mydata$pvalue[mydata$padj <=< 0.05])
Then you can adjust your plots however you like (presumably with some horizontal lines at the various alphas). Whether you take the smallest non-significant value or the largest significant value is up to you, just describe what "dots on the line" represent.