some details.

The white plotting area of the white board is about 22" by 34".
I first tried using a board about a yard long with the motors about the even with the edge of the white area.
I don't have any good photos yet, just some poor cell phone camera shots. I never took one of just the pen holder. It is made from KNEX.
I will be showing better pictures and maybe drawings of things later.

This is a lousy shot of the Basic Stamp board. There is one connector for each motor and one with three wires for serial communications.

Here are a couple shots showing how I glued sewing machine bobbins to stepper motors to wind the fish line to.

This is very blurry shot of one of the driver boards. I will have more details of those to follow although I have seen some circuits I would like to replace them with.
At first I just sat the board on the edge of a card table with the white board leaning up against the leg of the table. That very soon showed some serious problems so I built a frame to hold everything together. I built the top of the frame that holds the board with the motors longer so I could put some cuphooks at the ends to guide the string to the bobbins. This made the string wind much more evenly. That was the cause of the gaps in the picture on the first post. Here is what it looked like.

It still had a couple problems. The steps in the upper part of the board are less accurate than the steps in the lower part just due to the geometry of the thing. Also the pen has some friction to the board and this made the lines drawn going down not at accurate. That is why the upper circle in this picture has kind of a lump to the left. By the way this is a polar plot of several different phases of standing waves.
I do need to give credit to some of my picture ideas to Bruce Shapiro at taomc.com. This one is similar to one he calls 4ballo drawn by Sisyphus III.
http://www.taomc.com/art_machines/sisyphus/sisiii/4ballo1_800.JPG
To help solve these two problems I moved the board to the bottom of the legs of the frame and added the weight of a couple of dead AA batteries to the pen.