title: The XY coordinates in a d3 drag event are inverting
body: <p>I have a force layout with the following structure:</p>
  
  <pre><code>&lt;svg width="300" height="220"&gt;
      &lt;g class="scaleWrapper" transform="scale(0.3)"&gt;
          &lt;g class="transformWrapper" transform="translate(-110, -80)"&gt;
             &lt;g class="backgroundWrapper"&gt;
                 &lt;rect class="backgroundRect" width="300" height="220"&gt;&lt;/rect&gt;
             &lt;/g&gt;
             &lt;g class="forceNode"&gt;&lt;/g&gt;
             &lt;g class="forceNode"&gt;&lt;/g&gt;
             &lt;g class="forceLink"&gt;&lt;/g&gt;
          &lt;/g&gt;
      &lt;/g&gt;
  &lt;/svg&gt;
  </code></pre>
  
  <p>I also have a drag behavior linked to the rect to drag it around (scaling is handled through a separate slider).</p>
  
  <pre><code>        let transformElement = d3.select('.transformWrapper');
          let svgBackground = transformElement.append('g')
              .classed('backgroundWrapper', true);
  
          function originFunction() {
              let d = d3.select('.transformWrapper');
              return {
                  x: d.attr('x'),
                  y: d.attr('y')
              };
          }
  
          let svgDrag = d3.behavior.drag()
              .origin(originFunction)
              .on('dragstart', function(){
                  d3.event.sourceEvent.stopPropagation();
              })
              .on('drag', function(){
                  transformElement.attr("transform", `translate(${d3.event.x}, ${d3.event.y})`);
              });
  
          svgBackground.call(svgDrag);
  </code></pre>
  
  <p>It mostly works, but it jumps around as i drag it.  I did a log and saw that the d3.event object is alternating between relative XY coordinates and absolute ones, here's a sample of what I'm seeing:</p>
  
  <pre><code>-111 -80
  -29 -6
  -110 -80
  -29 -5
  </code></pre>
  
  <p>I don't see any other elements that have behavior bound to them.  All the d3.event objects have the 'drag' type property and the same srcElement.  How can I silence the events that are returning the relative positions?</p>