I believe that this Data Explorer query answers your question: "Home site" of election voters. The SQL is monstrous, but it basically just looks at voters in the election and identifies which site they have the highest reputation on. (I'm not sure how it behaves when it comes to picking a site when a user has the same, highest rep on two sites, but that's an edge case and should be rare. Also, the total number of voters reported by this query will not necessarily agree with the count at https://math.stackexchange.com/election because some voters may have deleted their accounts at some point in time after the election happened.)
You can examine the data in detail on the query's page; here are the top 10 sites:
The results are much as you would expect - a large majority from this site; a large minority from SO; and then a long tail starting with sites on topics of interest to mathematicians.
The underlying data in
#out is a list of (SE-wide account ID, site name, reputation on that site), for all users that voted in this election (as identified by having a Constituent badge awarded since the election started). If you want to slice and dice it in some other way (for example, bucketing by rep on Math.SE or something), you probably only need to edit the query at the bottom (beneath
-- Reap results) - though, obviously, you'll need to know SQL. If you want to do something more complex, let me know and I'll take a look.
For comparison, here are the same data for the 2013 election:
Also, I noticed that some users wanted additional data about meta participation of voters and so forth. If any of you have specific ideas about what kind of stuff you'd like to know, I can probably pull something together (provided, of course, that the necessary information is exposed in the Data Explorer).