Default graphical object setting. It is also a combo button: the other types of graphical objects can be selected after clicking on the black triangle in the lower-right corner of the button.

What kind of graphical objects are available in Marvin JS?

How to draw a graphical object

To draw a shape, choose the Rectangle or Ellipse tool, then drag the mouse cursor until the shape reaches the desired size.
Grey feedback shows what the real box will look like.

The size, shape and color of the object can be changed afterwards as well.