destroying elements

Sometimes you have the opposite problem from creation: you have too many existing elements, and you want to remove them. You can select nodes and remove them, but more commonly, you’ll use the exit selection to let D3 determine which elements are exiting the stage. The exit selection is the opposite of the enter selection: it contains all elements for which there is no corresponding data.

var circle = svg.selectAll("circle")
      .data([32, 57]);