The graph shows what happens when the diffusion is started. After some time, the concentration has become equal on both sides and net diffusion will be stopped
Why 'net'?) Answer: In reality, because the molecules are still moving according to their kinetic energy, some molecules will accidentally cross the membrane and move to the other side. But other molecules that are moving accidentally in the opposite direction will offset this. In other words, the molecules are still moving left and right but the net effect is zero.