code: public void onMouseDown(MouseDownEvent event) {
  		if (event.getNativeButton() != NativeEvent.BUTTON_RIGHT) {
  			dragging = true;
  			center = getScreenPosition(event);
  
  			LineString radiusLine = mapWidget.getMapModel().getGeometryFactory().createLineString(
  					new Coordinate[] { center, center });
  			mapWidget.getVectorContext().drawGroup(mapWidget.getGroup(RenderGroup.SCREEN), circleGroup);
  			mapWidget.getVectorContext().drawCircle(circleGroup, "outer", center, 1.0f, circleStyle);
  			mapWidget.getVectorContext().drawCircle(circleGroup, "center", center, 2.0f, circleStyle);
  			mapWidget.getVectorContext().drawLine(circleGroup, "radius", radiusLine, circleStyle);
  		}
  	}
docs: Register center point for the circle, and start dragging and rendering.
  
  @param event
  event