It’s not possible to reset analytics tracking.
You can, however, view only a particular segment of the graph, showing the period you are interested in.
If your heart is set on clearing all of the analytics data, you may want to consider cloning your board and moving on to using the clone instead of the original. When you clone a board, you have the possibility to copy all of its active (non-archived) tasks. However, the task history - including task comments, assignments and all of the analytics - will not be carried over to the duplicate.