d3 doesn't always include everything that we want to use in our maps, in this case the legend

This line of code imports the library to make legends

Import the simple statistics library. What you use to create natural breaks.

You can move your code out of order, it does not need to be in the correct order for it to execute correctly. You need to focus on making relationships.

Can convert numbers into different formats so that they display the way you want them to on your map. (ex. only having one decimal place)

Import the topojson library so it can read the topojson files