Would it be possible to place a background grid in the Superposition of Two Waves applet?  The center-line, or rest position of each wave would have to lie on one of the grid lines.  I want students to compare the amplitudes of the two original waves vs. the sum of the two waves quantitatively as they change the phase difference between them.  I have been using your simulations in class for close to 10 years, and I greatly appreciate all the work you have put into them.