Abstract: Presented in four different aspects is a plotter pen of the type utilized in an X-Y plotter mechanism controlled by computer to select a given plotter pen from a group of such pens and to mount the selected pen in the mechanism in position of use. In one aspect there is presented a universal cartridge for containing a marking fluid, which may be used with different marking "points" or "nibs", and which is expendable when the marking fluid in the cartridge is exhausted. In another aspect, there is presented a plotter pen that utilizes a disposable cartridge containing a marking fluid and which is equipped with a marking fluid flow control mechanism or system to control the flow of marking fluid from the plotter pen. In still another aspect, there is presented as an article of manufacture a marking fluid flow control mechanism for attachment to a disposable cartridge containing marking fluid to be controlled.