Using the enter selection, we can create new circles for any missing data. Each new circle is already bound to the data, so we can use data to compute attributes and styles:

var enter = circle.enter().append("circle");
  
  enter.attr("cy", 90);
  
  enter.attr("cx", 160);
  
  enter.attr("r", function(d) {
    return Math.sqrt(d);
  });