To show both opened and closed tickets, you'd use a report with two datasets as others have mentioned.
Reporting wont show a date for which no records are opened or closed. Logically, that date does not exists in the tables you are reporting on.
For this requirement, I would recommend using Performance Analytics. Since PA collects a score daily, there will be a score of 0 for those dates when no tickets were opened or closed. Simply set up two indicators, one for opened and one for closed, and show them in a time widget. Dates with a score of 0 will be shown in the x axis.
If you really want to do this without using PA, you could use database views. You'd need to create a custom table with one row for each date, and probably a scheduled job that runs at midnight and adds a new date row to the table. Then join your task to that table twice, once for closed_at matching the date and once for opened_at matching the date.
If you use left joins for the task tables, you'd still end up with a row for dates with no task records matching. Then you could use two data sets grouped by the date field of your custom table's date field from the DB view, each with a count of the closed and opened task sys_id columns grouped by date. This could work in theory, but it's not something I'd recommend. PA would be much better.
