What evidence does emerson use to support his claim that to be great is to be misunderstood

He claims two things: One that small minds are akin to a "shadow on the wall" and that there is no benefit in trying to please them because one never will.

Secondly, he asserts that other great men, like Luther, Socrates, Jesus, Galileo and Copernicus were also misunderstood, so being misunderstood isn't a bad thing. One should speak his mind fully and firmly and, if misunderstood, recognize that he is in great company.