It would be helpful to delete completed tasks in Planner which are 30 or more days old or be able to select completed tasks to be deleted.
The following is my thought process for tasks that are > 30 days :
The following is my thought process for selecting and deleting specific completed tasks:
Eventually, it would suffice to just have the option of removing the completed tasks from the overview graph. So you can keep the history, but the color of the completed tasks will not slowly take over the whole circle.
Check the workaround here: