You can add lines and other drawing objects only to a chart. You cannot add them to the datasheet.
On the Drawing toolbar, click AutoShapes, point to Lines, and then click the line style you want.
Drag to draw the line.
To constrain a straight line to draw at 15-degree angles from its starting point, hold down SHIFT as you drag.
To lengthen a straight line in opposite directions from the first end point, hold down CTRL as you drag.
Tip If you want to draw a simple straight line, click Line on the Drawing toolbar.