Crossfilter reduce :: find number of uniques

Question

I am trying to create a custom reduce function for a dataset attribute group that would sum a number of unique values for another attribute.

For example, my dataset

Answer 1

UPDATED ANSWER

Sorry I misunderstood the question... you are actually on the right track. You'll just need to maintain a count of each project so that your subtract function can know when to remove the value.

teamMemberGroup = teamMemberDimension.group().reduce(

        function (p, d) {
            if( d.project in p.projects)
                p.projects[d.project]++;
            else p.projects[d.project] = 1;
            return p;
        },

        function (p, d) {
            p.projects[d.project]--;
            if(p.projects[d.project] === 0)
                delete p.projects[d.project];
            return p;
        },

        function () {
            return {projects: {}};
        });

Here is an updated fiddle: http://jsfiddle.net/djmartin_umich/3LyhL/