3ddipole

3ddipole makes marbles.

Get it here: 3ddipole.

It's a shell script (see Scripting Basics). Give it a file of target coordinates and it'll make an overlay consisting of a set of spheres (with given radii and colors) that you can use with AFNI's 3d renderer.

You need to use the cutout feature of the renderer in AFNI to produce a picture like that. All 3ddipole does is make the marbles.

And no, I won't make it produce an arrow.

Unless somebody can come up with a 3dcalc expression that draws an arrow.