J-Walk and The Skeptical Optimist have been discussing charts lately. Methods in Excel even has a recent post about charting, but I doubt there’s any controversy over his findings. It’s high time I offered my unsolicited opinion.
Steve linked to an SAP guide to charts, which I hadn’t seen before but which looked interesting. I generally agree with everything on that page. If the data is showing proportions to a whole, a pie chart is better suited than a bar chart, in my opinion. When I see a pie chart, I know that the circle represents the whole of something and that each slice is someone or something’s share of it. I don’t think that’s abundantly clear with a bar chart. Therefore, I think a pie chart is most appropriate for the national debt data.
What’s not appropriate for any pie chart is too many data points. I think the limit is about six. More than that and it’s too hard to tell what the proportions are.
The first question that you should ask yourself when creating a chart is “What am I trying to sell and to whom am I trying to sell it?”. Not trying to sell something? Then why make a chart. You may be trying to convince you readers that the foreign owned portion of the national debt is not as high as they may have heard. You may be trying to convince your boss that your budget overages were necessary. You may be trying to convince yourself that you can afford that new tablet pc.
I like the pie-to-bar chart that J-Walk made, but the bar portion is unnecessary for this application. The sell job is the proportion of foreign owned debt, not how much is owned by Japan or China – so don’t put that information on there. If you think it’s interesting, make another chart. Here’s my proposal:
As you can see, my chart formatting skills are lacking, so consider this a prototype. Thoughts?