so I have this bunch of code:

MONGO_PIPE = [
    {
        "$match": {"dt": {"$gte": start, "$lte": end}},
    },
    {
        "$group": {
            "_id": {"$dateTrunc": {"date": "$dt", "unit": unit}},
            "value": {"$sum": "$value"},
        },
    },
    {"$sort": {"_id": 1}},
    {
        "$group": {
            "_id": None,
            "dataset": {"$push": "$value"},
            "labels": {
                "$push": {
                    "$dateToString": {"date": "$_id", "format": "%Y-%m-%dT%H:%M:%S"}
                }
            },
        }
    },
    {"$unset": "_id"},
]

In few words: it aggregates date, that matches range of dates start-end, and grouped by unit (day, week, hour, whatever)
My output now if unit=day:

{
"dataset": [1, 2, 3],
"labels": [day3, day4, day5]  #example dates
}

What I need is to generate zero values for day1, day2. There are no documents matching dates day1 and day2. So output should look like this:

{
"dataset": [0, 0, 1, 2, 3],
"labels": [day1, day2, day3, day4, day5] #example dates
}

I know that $match is insufficient here, and I need to generate offset based on unit.
I am like 5 hours in this DB for the first time.
If somebody knows the answer – please let me know, thanks in advance