Proportional circle maps show point data and are mapped with a circle instead of a dot. The size of circle relates to the measured variable and not necessarily the area over which it is measured. In this case, the variable measured is volume of products (cargo) shipped (in thousands of tons) from the largest ports in Europe in the mid 1850s. The map maker includes all ports that carry over 200,000 tons of cargo per year and indicates its actual amount on the dot.
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