binding data

This is beginning to look a lot like jQuery. More commonly, though, we want to use data to drive the appearance of our circles. To do that, we need some data. For the sake of example, let’s imagine that each of these circles represents a number: 32, 57 and 112. The data operator binds these numbers to the circles:

circle.data([32, 57, 112]);
  

All data in D3 is specified as an array of values. Conveniently, this mirrors the concept of a selection, which is just an array of elements. Notice then how the first number (the first datum, 32) is bound to the first circle (the first element, on top), the second number is bound to the second circle, and so on.