I think you are misinterpreting the chart. It doesn't show who atheists would vote for. It does show that atheists are less popular than gays.
BTW I'm an atheist and would vote for an atheist.
Even if you don't have the gift of faith, for America's Commander in Chief you want a leader that has the God advantage. An atheist commander facing poor odds gives up quicker. The evolutionary purpose of religion is it provides a competitive advantage during tribal warfare. Christianity has the best track record for that.