How do you evolve graveler in Pokemon Platinum?

To evolve Graveler in Pokemon Platinum, go to any Pokemon Center. Go upstairs. Go to

the second lady at the counter. This gets you to the Union Room. If any other person [besides the creepy lady in the corner] is in there, talk to him/her. Do a pokemon trade with them. Trade Graveler with the other person's pokemon of their choice. After the trade, Graveler will evolve. Trade back the other person's pokemon and Voila! you have a Golem!

Note: You cannot battle or trade with the creepy lady in the corner. She has no pokemon.