Update-Only Transition

Demonstrating how to apply a transition to only updating nodes, and not entering nodes.

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var width = 960,
    height = 500;

var x = d3.scale.ordinal()
    .domain([0, 1, 2])
    .rangePoints([0, width], 1);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

var circle = svg.selectAll("circle")
    .data([0, 1])
  .enter().append("circle")
    .attr("r", height / 4)
    .attr("cx", x)
    .attr("cy", height / 2);

setTimeout(function() {
  circle = circle.data([1, 2], function(d) { return d; });

// Since this is created before enter.append, it only applies to updating nodes.
  circle.transition()
      .duration(750)
      .attr("r", height / 3)
      .style("fill", "orange");

circle.enter().append("circle")
      .attr("r", height / 4)
      .attr("cx", x)
      .attr("cy", height / 2)
      .style("fill", "green");

circle.exit().transition()
      .duration(750)
      .style("fill", "red")
      .attr("r", 1e-6)
      .remove();

}, 1000);

</script>

https://d3js.org/d3.v3.min.js