There are a number of options, but it depends on the format of your data and how exactly you're trying to graph this. If each of these are actual columns, you could use something like:
SUM(IF [Beer] = 'X' THEN 1 ELSE 0 END) / SUM([Number of Records])
This should give you a 1 for each type of alcohol consumed in the month divided by a 1 for every row within the month, which (theoretically) is all people. Does that help?
Wow, I was WAY over-complicating this. That will do the trick. Thanks!