Hello,
In my polargraph i use this fantastic pen: KOH-I-NOOR Fiber professional

It is available in different sizes from 0.1mm and larger. Here in Italy it costs 2,40€. a little expensive, but it is very durable. With one pen i made 5-6 A4 drawings and it still works. The real size of the line is 0.3-0.4mm with 0.1mm pen on normal inkjet paper.
The smallest size (0.1mm) is only available in black color. Red, blue and green are available from 0.4mm, but I did not even try.

PROs:
- very fine tip
- durable
- not excessive leaks on paper

CONs:
- a little expensive
- 0.1mm only available in black