This example shows the very basics of the D3 data binding system, which creates selections via data joins.

A set of circle elements are initialized with data, which creates the three selections:

• The enter selection: elements that did not previously exist.
• The update selection: elements that currently exist.
• The exit selection: elements that exist, but have no longer have data.

In this code, for each element in the enter group, we append a new circle element. We then calculate its properties from the data that has been joined with it.

For a full explanation and demo, see the General Update Pattern article.