In this chapter we introduced the concept of using data-joins in D3. A data-join allows you to make elements enter a web page, change, or update, once they are on the page, and exit. And it allows you to do all of those things based on data. This chapter covered in detail how to use a data-join to make elements enter the page. And it also expanded on our population bar chart example, showing how to build the entire thing using data-joins.