On the x-axis, are the number of answered quesitons. On the y-axis, it's the number of students. So as an example, let's assume that our exam has 10 questions. So when a student has answer 5 questions, it will show him/her in the 5th position on the x-axis. As he/she continues answer other questions, then we can see the progress until he/she completed all the exam questions.