As its name suggests, the Straight Line Tool is used to draw straight lines. To draw a straight line, click and hold down the left or right mouse button where you want the line to begin. Drag the mouse cursor to where you want the line to end and release the mouse button. A straight line connecting these two points will be drawn.
The left mouse button will draw the line using the color on the left side of PixelNEO's Color Box. The right mouse button will draw the line with the color on the right side of the Color Box.