A common way to visualize the layer on the map is by being able to specify a unique color for a specific value or range of values, and for that layer to be able have multiple ranges defined.    For example, let's say you were displaying the likelihood of forest fires in different states in the United States.  You may have a 0-20, 21-40,41-60,61-80, and 81+ as the five ranges.   The map would need to be able to render each feature (polygon or line or point) based upon a value in the incoming dataset (in geojson, it could be a attribute field with the value, or it could be a property within an enumerated list).    Another case would be an electoral results map, showing who one each county.   One color would symbolize Republican, another Democrat, and a final "Other" (which doesn't seem to occur).   Being able to do this is a basic map visualization, and to also be able to display these ranges/values in the legend would be helpful.