code: function(d) {
              return function(t) {
                  var c = d.sets.map(function(set) {
                      var start = previous[set], end = circles[set];
                      if (!start) {
                          start = {x : width/2, y : height/2, radius : 1};
                      }
                      if (!end) {
                          end = {x : width/2, y : height/2, radius : 1};
                      }
                      return {'x' : start.x * (1 - t) + end.x * t,
                              'y' : start.y * (1 - t) + end.y * t,
                              'radius' : start.radius * (1 - t) + end.radius * t};
                  });
                  return intersectionAreaPath(c);
              };
          }
docs: interpolate intersection area paths between previous and current paths