title: Tweening numbers in D3 v4 not working anymore like in v3
body: <p>I'm trying to figure out why my tweening numbers (counting up or down) code in Version 4 of D3 doesn't function any more.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>var pieText = svg4.append("text")
      .attr("class", "pieLabel")
      .attr("x", 0)
      .attr("y", 0)
      .text(0)
      .attr("dy", "0.2em")
      .style("font-size", 19)
      .style("fill", "#46596b")
      .attr("text-anchor", "middle");
  
  d3.selectAll(".pieLabel").transition()
    .delay(500)
    .duration(1000)
    .tween("text", function(d) {
      var i = d3.interpolate(this.textContent, d.value);
      return function(t) {
        this.textContent = form(i(t));
      };
    });
  </code></pre>
  
  <p>The console.log tells me that the interpolation works fine. 
  So what has changed? And how do I get it to work?</p>
  
  <p>Thanks for your help.</p>