In a plotter or other device for drawing inked lines on a receiving surface, an ink reservoir holding a large quantity of ink is provided to attain a long operational life for the associated pen. The reservoir includes a piston or other moveable working member to which air pressure is applied for pressurizing the ink, and the ink reservoir and associated pen are permanently connected with one another and with a few ancillary parts to provide a unit assembly readily connected to and disconnected from a pen head to permit quick exchange of units without danger of ink spillage. The ink reservoir is transparent or translucent to give the operator a visual indication of when the unit assembly is in need of replacement because of imminent ink exhaustion, and sensor may also be provided for automatically producing an electrical "out of ink" signal which appears upon the complete exhaustion of ink from the unit.