There are a few ways you could get this effect, although it isn't directly supported by the chart's datapoints. Here are two of them:
Method 1) Enlarge your data points so that their radius is as large as you see on the picture. Then add an annotation on the datapoint so that the text is positioned within the circle.
Method 2) Add an annotation onto your datapoints, where the annotation is a UIView consisting of the circle you want and a label.