Seth Godin, marketing guru and well-known author of numerous business books, recently wrote a pair of blog posts that caused something of a stir among "data presentation purists," by asserting, among other things, that it would be better if people NEVER used bar graphs in powerpoint presentations.
Actually his first post, The three laws of great graphs, is less about bar graphs specifically than it is about the effective use of graphs (or charts, he uses the terms interchangeably) in powerpoint presentations. In particular, one of Godin's three laws is "No Bar Charts," and he states:
The only reason (did I mention only) to use a chart in a presentation is to make a point. If you want to prove some deep insight or give people textured data to draw their own conclusions, DON'T put it in a presentation.
While Godin later clarifies that he is talking about the use of charts and graphs in presentations to non-scientists, I still think this is overstated. I'm not convinced that non-scientists are unable to handle deep insight or draw their own conclusions from a presentation. The best way to make your point (or make your point clearer) will depend on a lot of things, including your topic and your audience, but it will also depend on which method of data presentation (e.g., a bar graph or pie chart) you choose.
In Godin's second post, Bar graphs and pie charts, he sings the praises of the pie chart while again denigrating the bar graph, and this post I must take issue with. Bar graphs and pie charts can both be effective, but they are intended for different purposes.
If you have a number of items that all add up to 100% of a whole "pie" and you want to show the relative size of each of the items, a pie chart is often your best bet; it's simple, clear, and easy to understand. Stephen Few has written a couple of good posts (here and here) in which he talks about the usefulness of pie charts for this type of part-to-whole comparison.
Often, however, you want to show the relative size of a number of items, yet the items are not parts of a whole. In this instance, a pie chart doesn't make any sense, while a bar chart may be just the ticket.
So while I join with Mr. Godin in his praise of the simplicity and usefulness of the pie chart, I must disagree with his harsh criticism of the bar graph's value in presentations; as in many other situations, one should first decide what information it is that one wishes to convey, and to whom, and then choose the best tool for the job.