Post Made Community Wiki by whuber
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We could stay here all day denoting best practices, but you should start by reading Tufte. My primary recommendation:

Keep it simple.

Often people try to load up their charts with information. But you should really just have one main idea that you're trying to convey and if someone doesn't get your message almost immediately, then you should rethink how you have presented it. So don't start working on your chart until the message itself is clear. Occam's razor applies here too.