@Joe: Yes, pens arrived this morning. I've got about 7 black ones now that either work or look like they should work because they've never been initialised (you have to screw the nib part into the ink reservoir and it punctures a hole into a sealed-off ink chamber). I've also got four coloured ones, and another 4 clogged-up originally black ones - but I suppose I could try different ink in those if / when I get them cleaned out.
@Ari: You can see from the attached photo (two four-packs showing the top of the coloured pack and back of the black pack) that the plotter uses special pens - but the 'writing hand' part that grips the pens only holds them by the 'collar' and steadies them down near the nib. It might be possible to 3D-print a collar to slide over some ordinary pens so that they fit. Another restriction is on the height (length) of the pens, but you could get around that by removing the top cover of the plotter, or sawing a slot in the top cover - it makes a sort of bridge that the pens pass under when moving from the pen storage carousel to the plotting bed. I've put an ordinary Sharpie in the photo for comparison - that would hit the 'bridge' even if it would fit in the holder.
The plotter manual says it recognises four different types of pen by looking at 'bands' around the top part - and you can set up different plotting speeds and how much downward pressure is applied to the pen when writing for each of the types. I've not looked in to that yet as all the pens I have are the same 'ink on paper using a Radiograph-style nib'. You could also get ball-point, felt-tip and some other variety back in the day.
