Here is a graph that shows the price of blueberries at two different stores. Which store has a better price?

Two line graphs. Weight in pounds. Cost in dollars. Horizontal axis, 0 to 5, by 1's. Vertical Axis, 0 to 40, by 10's. First line begins at origin, trends upward and right, passes through 1 comma 6, 2 comma 12, 3 comma 18, 4 point 5 comma 27. Second line begins at origin, trends upward and right, passes through 1 comma 4, 3 comma 12, 4 point 5 comma 18. 

We can compare points that have the same $x$ value or the same $y$ value. For example, the points $(2, 12)$ and $(3, 12)$ tell us that at Store B you can get more pounds of blueberries for the same price.

The points $(3, 12)$ and $(3, 18)$ tell us that at Store A you have to pay more for the same quantity of blueberries. This means Store B has the better price.

We can also use the graphs to compare the constants of proportionality. The line representing Store B goes through the point $(1, 4)$, so the constant of proportionality is 4. This tells us that at Store B the blueberries cost $4 per pound. This is cheaper than the $6 per pound unit price at Store A.