When dealing with changing data, we typically create an update() function, to which we pass new data.

We re-bind the new data to the element and recalulate its properties. This lets us separate the data update logic from the presentation logic. In D3 terms, we're updating the data, and letting D3 drive the updates to the document.

We're also introducing the d3.interval() function here, which is similar to the window.setTimeout() function in javascript, but with some additional functionality that we're not using yet.