Animate a dot with a few transitions

<!DOCTYPE html>
<html>
<style>

circle {
  fill: blue;
}

svg {
  background-color: parchment;
}

</style>

<body>

var points = [{start: 10, mid: 30, end: 50, id: "A"}];

var width = 500;
var height = 200;

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

// make a scale for the min start and max end as your domain
var locScale = d3.scale.linear().range([10, width-10]).domain([20,50]);

var circles = svg.selectAll("circle")
  .data(points, function(d) {return d.id;});

circles
  .enter()
  .append("circle")
  .attr("cx", function(d) {
    return locScale(d.start);
  })
  .attr("cy", height/2)
  .attr("r", 10);

circles.transition()
  .duration(1000)
  .attr("cx", function(d) {
    return locScale(d.mid);
    })
  .transition()
  .duration(200)
  .attr("r", 30)
  .transition()
  .duration(200)
  .attr("r", 10)
  .transition()
  .duration(1000)
  .attr("cx", function(d) {
    return locScale(d.end);
  });

circles.exit().remove();

</script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js