The problem you are going to have hear is counting the number of tasks on each date as every date is not included in your data set.
You can only aggregate on the data that is available.
To get around this you will need to create a separate table with a list of all dates that you are required to report on.
You can then link this if it created in same datasource or use it as blended data.
If you share a sample workbook I can help you achieve this.