I linked to a great post on FlowingData a few weeks ago about some basic rules for visualizations. Nathan then gave a more detailed treatment of his first rule:

I already covered the small handful of rules that pertain mostly to traditional statistical graphics. The first one–to always start your bar charts with a zero baseline–unexpectedly drew some disagreement, and I am unexpectedly compelled to go into more depth.

Great alternatives to a non-zero baseline.