title: Focus+context via brushing mouseovers zoom issue
body: <p>Complete code is in the <a href="http://jsfiddle.net/67tpvh16/2/" rel="nofollow noreferrer">jsfiddle</a>.
  The problem is when zooming mouseovers in the top and bottom chart do not match, i.e. moving circles in the bottom should not go beyond the area that is selected via brushing. I'm updating the moving points as follows:</p>
  
  <pre><code>  focus.on "mousemove", () -&gt;
       xPos = d3.mouse(this)[0]
       updateMovingPoints(xPos)
    context.on "mousemove", () -&gt;
       xPos = d3.mouse(this)[0]
       updateMovingPoints(xPos)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/YqekI.png" alt="enter image description here"></p>
  
  <p>As you see, the bottom mouseovers do not match with top chart, i.e. go beyond zoomed area.</p>