I assume you are referring to this example. In that case, MRect is actually a custom class of that sketch. You can make your own class like it or better yet, slightly adapt the MRect class to suit your needs. For example, you could change the line...

1.       rect(xpos+(i*(d+w)), ypos, w, height*h);

...into...

1.       ellipse(xpos+(i*(d+w)), ypos, 10, 10);

...and you should see a group of circles instead of rects. You can put smooth() in setup() to make it look a little smoother.

But like I said, you can write your own custom classes, just like MRect.