X
                                 X
                                 X
number of                        X
homeworks                  X     X
                           X     X
                           X  X  X
                           X  X  X
            X              X  X  X
            X  X           X  X  X
            -----------------------
            3  4  5  6  7  8  9  10
                    grades

The histogram shows the distribution of grades.