Visualization of Magnetic Field

I found a java applet for visualizing the magnetic field of a specified configuration of magnets on Paul Falstad's web site:

To use this applet, I first pressed "clear all."  Then I selected "Mouse = Add Magnet (Down)" and painted in the location of one magnet and similarly for another magnet (Up) and for a feromagnetic (iron) plate below.

The figure below shows orientation of the B lines.  Higher intensities are white and lower are green.