[{"date":"12-2-2015","cost":100},{"date":"13-2-2015","cost":10},{"date":"12-2-2015","cost":40},{"date":"13-2-2015","cost":30},{"date":"13-2-2015","cost":80}]

db.collection.aggregate([{"$facet":{"total":[{"$group":{"_id":null,"total":{"$sum":"$cost"}}}],"coll":[{"$group":{"_id":"$date","cost":{"$sum":"$cost"}}}]}},{"$unwind":{"path":"$coll"}},{"$project":{"total":{"$let":{"vars":{"t":{"$arrayElemAt":["$total",0]}},"in":"$$t.total"}},"date":"$coll._id","cost":"$coll.cost"}}])