Automap art is the practice of drawing lines that do not necessarily serve functional purposes for gameplay or first-person visuals so as to change the automap display. The functional lines delineating the level may be hidden away so as not to interfere with the drawing. In practice, it turns all or part of the automap into a simple piece of vector art. Automap art makes use of the "hidden" and "secret" linedef flags to alter their appearance, and may also use the "shown" flag to have the entire piece revealed from the start without needing to explore.
In vanilla Doom, there are four colors available for automap art: gray for unseen lines revealed by a computer area map powerup (or equivalent) red for single-sided lines (or lines with the "secret" flag), yellow for ceiling height changes, and brown for floor height changes. Ceiling height changes can be hidden visually in first-person mode when the ceiling is the sky, in all cases the effect can be minimized by using only one unit of height difference, as that is enough for the automap.
Examples of automap art include Grove, which replicates the look of a hand-drawn map, and MAP08: 54-pit from the Japanese Community Project which features a comic page about Doomguy's adventures within the map.