A boxplot is a standardized way of displaying the distribution of data based on a five number summary minimum, first quartile, median, third quartile , and maximum. It can tell you about your outliers and what their values are. It can also tell you if your data is symmetrical, how tightly your data is grouped, and if and how your data is skewed.
If you have a grid with a hundred units and you need to show 1 hundredth (or 1%) you shade in 1 unit. If you need to show a tenth (or 10%) you shade 10 Units. I would need a picture of the grid to help any further.
