3.3 Class diagram
The class diagram of the Circle and Point classes and their relationship now looks like this:
+ means public
- means private
<<constant>> means constant field (PascalCase per C# conventions)
Underlined means static
Each Circle references one Center (Point); a Point may be shared by several circles (see 3.4 The relationship between Circle and Point).
This is design 2. Why the UML is an association (not composition), and how it differs from storing centerX/centerY, is explained in Three ways to give a Circle a centre.