A pen plotter includes a means for applying pressurized air to the ink reservoir supplying the pen with ink. A pressure sensor associated with the pressurized air space provides an out-of-ink signal when the reservoir and pen become empty and the air pressure becomes lost due to air leakage through the pen. The plotter may be one in which a web of paper is intermittently advanced to bring fresh portions of the web to a support surface cooperating with the pen, and in which a pump is used to supply the pressurized air. The pump is driven by the same motor as drives the means for advancing the paper so that the pump is operated only during paper advancements. The pump is of a simple low cost construction and is adjustable to vary the pressure maintained in the ink reservoir.