title: D3 v4: Update force layout
body: <p>Mike Bostock <a href="https://bl.ocks.org/mbostock/1095795" rel="noreferrer">has an example</a> regarding updating a force layout. The example is based on v3 — how can the same functionality be replicated in v4?</p>
  
  <p><a href="http://bl.ocks.org/DavidChouinard/cc2a55d310bee68900eebb822ba2344b" rel="noreferrer">Here's my (pitiful) attempt</a>.</p>
  
  <p>I've read <a href="https://github.com/d3/d3/blob/master/CHANGES.md#selections-d3-selection" rel="noreferrer">the changes</a> to selections in the v4 Changelog, but the <code>merge</code> call is still confusing. In particular, it's not clear to me how the data join interacts with the simulation <code>nodes()</code> and <code>links()</code> call.</p>