A TickMarks object defines a set of tick marks for one axis. Tickmarks may extend an adjustable distance (given in view coordinates) inside and outside the graph frame (including all the way across the graph, for making grid lines), and may have labels. By default, numeric labels will be generated based on the axis range.
In a 3D graph, each tick mark is actually two lines, extending in the two directions orthogonal to the axis. For example, tick marks on the X axis extend both in Y and in Z.