instruction: Return the completion date for all the tests that have 'Fail' result, and count them by a bar chart, list from high to low by the Y-axis.
response: SELECT date_of_completion, COUNT(date_of_completion) FROM Student_Course_Enrolment AS T1 JOIN Student_Tests_Taken AS T2 ON T1.registration_id = T2.registration_id WHERE T2.test_result = "Fail" GROUP BY date_of_completion ORDER BY COUNT(date_of_completion) DESC