title: Can't transpile typescript due to d3 event error
body: <p>I have a d3 scatterplot chart in an Angular 2 project using typescript and webpack. Part of the chart responds to a mouseover event to display a tooltip. Now the mouseover event actually fires perfectly well, however I see an error when webpack transpiles the typescript.</p>
  
  <pre><code> Property 'pageX' does not exist on type 'Event | BaseEvent'.
  </code></pre>
  
  <p>Here is a snippet showing the mouseover part of the chart element and the offending <code>d3.event.pageX</code>.</p>
  
  <pre><code>  .on("mouseover", (d: any) =&gt; {
      if(d.roleName) {
        div.transition()
          .duration(200)
          .style("display", "block")
          .style("opacity", 1);
        div.html(d.roleName + "&lt;br/&gt;"  + d.org.name)
          .style("left", (d3.event.pageX) + "px")
          .style("top", (d3.event.pageY-80) + "px");
      }
    })
  </code></pre>
  
  <p>I am relatively new to typescript but gather the above issue may be something to do with the d3 typings file not mapping correctly?</p>
  
  <p>Any guidance greatly appreciated.</p>