If you have the bytes per day as a number you can store this as a value inside an RRD.
The linked article describes how to calculate the complete sum over a time range.
In the end it is build the average over all values in the time range and multiply with the amount of intervals (days). If you do it manually it looks like this.
| Day | 1 | 2 | 3 | 4 | 5 | 6 | 7 | Sum |
|---|---|---|---|---|---|---|---|---|
| Bytes | 2000000 | 3000000 | 6000000 | 1000000 | 12000000 | 5000000 | 100000 | 29100000 |
| Average of all days | 4157143 | |||||||
| Total traffic = Average * 7 | 29100000 |
This is what you can achieve with RRD databases and RRDTool