[{"level":"6","x":2.4,"y":8.3},{"level":"6","x":0.1,"y":1.1},{"level":"6","x":9.4,"y":4.6},{"level":"4","x":3.3,"y":8.1},{"level":"4","x":2.7,"y":3.3},{"level":"4","x":6.3,"y":0.5}]

db.collection.aggregate([{"$lookup":{"from":"collection","localField":"level","foreignField":"level","pipeline":[{$group:{_id:"$level",min_x:{$min:"$x"},max_x:{$max:"$x"},min_y:{$min:"$y"},max_y:{$max:"$y"}}}],"as":"aggregation"}},{"$unwind":"$aggregation"},/** cosmetics / wrangling*/{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT",{max_x:"$aggregation.max_x",max_y:"$aggregation.max_y"}]}}},{$unset:"aggregation"}])