Imaginary numbers can be represented on a two-dimensional x/y graph, with the horizontal axis for real components and the vertical for imaginary components. However, there are numbers like quaternions that require a three-dimensional representation, utilizing an additional z axis for their imaginary components. Quaternions consist of one real part and three imaginary parts, exemplified by expressions such as w = 2 + 1i - 2j + 3k. No numbers exist that can only be graphed using the z axis alone without incorporating the x and y axes. Understanding these concepts is essential for visualizing complex mathematical structures.