If the drawing area is not a square, arranging the levels
as circles is not always the best choice. You can specify the aspect
ratio of the drawing area to better fit the layout to the drawing
area. In this case, the algorithm uses ellipses instead of circles.
See Tree layout in radial layout mode with aspect ratio
1.5 for an example.
To specify the aspect ratio:
Use the method setAspectRatio, whose value is a number. You
calculate the aspect ratio value from the width and height of the
drawing areas as aspectRatio = width/height:
treeLayout.setAspectRatio(1.5);