Throughout Candide, Voltaire uses an absurd tone and presentation of the story, which clearly incites laughter.  Yet the often exaggerated, outlandish, and senseless events in the novel force the reader to confront the overdone, unbelievable and irrational nature of the real world.  Thus, the tone of Candide, which is usually ironical and satirical, underscores the theme of the work as a whole.

More concrete examples of metaphor use obviously require a mention of the following:

Eldorado: Eldorado is the mythical city of gold spoken about by many authors before Voltaire's time.  It symbolizes the physical embodiment of utopian ideals and the limitless potential of human reason.

Pangloss' optimism: Leibnizic optimism is heralded by Pangloss throughout Candide as the perfect, harmonious ordering of the universe.  Among other things, it says that all things are for the best, that this world is the best of all possible ones.  Unfortunately for Pangloss and the rest of Candide's characters, experience proves time and time again that the earth is not a utopia, that needless, irrational suffering does occur even to good people.  In this way, Voltaire tears apart the deterministic optimism of Leibniz, Pope and others. 

Farm: Cultivating the garden is the final metaphor in Candide.  Candide and company decide to give up their philosophic ideals in exchange for productive practicality.  This change in focus shows that Candide has recognized the imperfection of his world and man's inability to comprehend let alone conquer the evil in his world.