The explanation of the ``box and whiskers'' plot is as follows: The top line reaches from the maximum grade down to the first quartile (that is, the top fourth of the class received grades in that range). The box shows the range from the first quartile down to the third; the middle half of the class got those grades. The bottom line stretches down to the minimum grade. The horizontal tick marks show the median, dividing the upper half from the lower half of the class. Finally, the central dot shows the mean, or average, grade on the assignment.
If you're curious, you can see similar graphs for individual problems on the midterm.
I don't include zeroes in any of the statistics.