What plotter do you have? I have an old Ioline 4000. You will be surpised at a solution I arrived at. I plot only occasionally, so liquid ink pens tend to dry out and become useless soon. The fiber tip pens are okay, but NOT if you are plotting detailed work, because the tip flattens out soon. I have tried roller ball pens, and they tend to skip a lot. One day, I was writing with a UNI-BALL micro pen by SANFORD (available at Office Max, Office Depot, etc.), and noticed how smooth and skip-free it was. The point is about .3mm. I wondered how it would do as a plotting pen. Alas, there was no way to mount it on the pen holder (a cup looking device with a hole in the middle for a plotting pen), or was there? The hole was about 1/8" larger than the pen. Being a good engineer, I improvised. I wrapped the pen with ordinary drafting tape several times, till I got a snug fit. I had to make sure the pen stuck out enough, so I measured it against a standard plotting pen in an identical holder. Then I tried it. IT WORKS! The pen just plotted away, no skips, no runs, no errors. I got better results than with roller ball or fiber tipped pens. The pens are available in several colors. Give it a try! Works for me.