title: d3.event.x does not get the x cursor's position
body: <p><br>
  I do not understand why d3.event.x == NaN and not the x cursor's position when I use d3.behavior.drag().
  I used d3.event.sourceEvent.offsetX instead, but it works only with Chrome (e.g. not with Firefox).
  The side effect can be seen here :
  <a href="http://gloumouth1.free.fr/test/Nature" rel="nofollow">http://gloumouth1.free.fr/test/Nature</a>
  The user can move the "frequency cursors" only with Chrome.<br>
  Any idea ?<br>
  G.</p>