Take the first picture as an example. The horizontal lines in the figure represent the components themselves. Diagonal lines and curves represent the changing trajectories of bending moment or shear force in different parts of the component, which are data graphs.

If you want to know the magnitude of bending moment or shear force at any point of the component, you can draw a vertical line from the corresponding position, and the value of the intersection of the vertical line and the data graph line is the answer.

It can be considered that the data graph is a function curve composed of vertical and horizontal axes, the horizontal axis is the distance from the left end of the component, and the vertical axis is the magnitude of bending moment or shear force.

Therefore, the second diagram can be used to represent the moment diagram and shear diagram, and the vertical lines in the diagram can more intuitively represent the corresponding relationship between each part of the component and the data diagram, and it is also convenient to distinguish the component from the data diagram.

The auxiliary lines of bending moment diagram and shear diagram have nothing to do with "shadow" at all, nor are they for aesthetics.

The diagonal line in the third picture is meaningless. It is wrong because it is meaningless.