I am kind of addicted to plotting with my CNC. It is so quiet, and the results are strangely satisfying. I am on the third version of pen holder for my machine now. I am quite pleased with the latest one I've made. I can change pens easily with it, and it appears to work well too. I settled on using Pilot Precise V5 extra fine pens. Because they're cheap, and easy to get. Though I kind of prefer how a good regular ball point works over these pens. Oddly getting plain ball point refills is not so easy these days. At least not the metal ones like the one pen holder I made needs. It worked pretty good, but I think my latest pen holder works better. Just these Pilot pens are not quite as good. I may have to figure out some way of using other ball point pen refills? When the pens I'm using now die I will look into how I can put plain ball point refills into them.
Here's another plot I did on my machine http://i.imgur.com/YjsaF5g.jpg Due to how the software I used was setup it ran over that 3 times. That's plain blue ball point ink. My CNC machine is like an artist.
Here's my latest pen holder http://i.imgur.com/KSlrLhH.jpg The pens are a friction fit into that tube. They just jamb up in there. Because the pen bodies are tapered, and I put a chamfer on the end of the tube.