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European Energy: Rounding and presenting data values in a legend
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What: The population data on our map was a secondary piece of information so we needed a legend that would be quick and easy to read. We used the following points to guide us:

  • Meaningless numbers in a legend will slow down map readers trying to understand your map.
  • A legend should only contain the information needed to quickly understand the map.
  • The information in the legend should be arrayed in a way that makes it easy to understand the nature of the information.

Why: For example the legend on the left is difficult to quickly understand because there are extra numbers, because the numbers are not meaningful, because the alignment of the numbers makes it difficult to compare and understand relationships between the rows. Whereas the legend on the right contains the minimum amount of information needed; it uses small numbers that are quickly understood; and the numbers are aligned to ensure comparability. Further for this map we felt readers only needed a rough idea of the populations of the countries.

How: To make the legend on the right, two significant steps were taken beyond what we used for the default legend on the left. First, we added a new long integer field the data and calculated it to be equal to the population field divided by 1,000,000. The result is a number that has been rounded to the nearest million. The data was then symbolized using this new field instead of the original population field.

The second step was to customize the legend labels. To do this, open the layer’s symbology properties. Edit the label by clicking on the text that is supplied by default and you will be able to type anything you like for a legend label.

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