Lineout mode is only available when the vis window contains 2D plots. A lineout is essentially a slice of a two dimensional dataset that produces a one dimensional curve in another vis window. When a vis window is in lineout mode, pressing the left mouse button in the vis window creates the first endpoint of a line that will be used to create a curve. As you move the mouse around, the line to be created is drawn to indicate where the lineout will be applied. When you release the mouse button, VisIt adds a lineout to the vis window and a curve plot is created in another vis window.