title: How can I sum binned time series using d3.js?
body: <p>I want a simple graph like:</p>
  
  <p><img src="https://i.stack.imgur.com/LMwwV.png" alt="financial graph"></p>
  
  <p>The data I have is a simple list of transactions with two properties:</p>
  
  <ul>
  <li>timestamp</li>
  <li>amount</li>
  </ul>
  
  <p>I tried d3.layout.histogram().bins() but it seems it only supports <em>counting</em> the transactions.</p>
  
  <p>I mustn't be the only one looking for that, am I ?</p>